In other words, you are thinking of "mercenary" as a full-time professional soldier for hire. This is specifically contradicted by the story, though.
Manga ch. 36.1 p.8:
In other words, in this world mercenaries are riff-raff hired to bulk out a fighting force; they are not skilled and likely not experienced.
Vice versa, the ones who are skilled probably aren't mercenaries by Prince Hubert's assessment, so there's no reason to think they would have the full-time mercenary life you are suggesting.
Of course, he could be wrong, but he's presented as quite skilled and knowledgable about military matters and the area, so probably not.
Note that the leader and the ones with him when he put down Lia and ignored her are probably in the skilled group, since they are directly involved in the kidnapping part.
Actually, that's exactly what I'm talking about with them not being implied to be more than mercenaries- All that implies is that they have better martial skill than a common mercenary, similar to the hunters; or, in short, they're professional soldiers-for-hire, rather than rabble sellswords, like they took on to pad out their ranks and engage the people in the back, who just had to be stopped from coming to support the front lines long enough that they could break through and capture Lia.
They're definitely in league with someone or someones who are more than meet the eye, but at this moment there's nothing beyond the leader shown as being apart from the men, ie. not a merc himself but a knight in service to a noble, or a higher-ranking minion of some cult, perhaps (considering that they had four large-sized barrier boxes themselves, there definitely is a lot of magical power behind them, but there's little likelihood it has anything to do with the Kingdom, so it's some great magic power holder or holders outside the bounds of the Kingdom that are likely behind it (and given the leader has golden eyes Lia can see in the dark, he's likely one, though not a noble given his simple orders and understanding of them in ch. 36), which is where the concept that it may be a cult comes from; Not likely a noble of Wester either, since they'd just be sabotaging their own situation, unless they have an illogically strong hate boner for the royal family and would rather see everything burn rather than Wester get better for everyone, including the royals). We can see that the elites with the leader have different clothing too, in ch. 35 and 36.1, dressed in robes that look black and more refined (as when we see the leader's outfit), versus the ones already fighting in lighter colours.
See above. That's exactly the sort of thing being implied. The fact that the leader is concerned about their faces being revealed, not to mention that a noble is implicated as being behind it, is also clue toward that.
See above; all that is implied is that they are very skilled, not your common merc nor a noble's personal guard. The leader is concerned about being revealed and being able to be tracked down, since they're attacking royalty and high-ranking nobles with the intent to kidnap; they cannot risk being known, obviously, as that would severely impact their efforts, as they would be able to spread the word to villages and such that they pass by, including guard posts, who can spread the word throughout the kingdom, and cause significant trouble for anyone seen and anyone in their company, too.
FYI, "bandit" is not what you would call a part-time mercenary. Bandits are defined by outlaw behavior; mercenaries are defined by soldiering for hire; neither has to do with frequency. One can be a bandit, a merc, neither, or both, and any of those can be part-time or full-time.
Yes, it is. Mercenaries fought for coin, and when they could not find work, they lacked coin to survive, so turned to banditry to survive until they could get a contract again, or made the switch full-time if they discovered banditry was more profitable; sometimes individually, sometimes working in bands. Again, this is very well-documented historically, and is why mercenaries were so well-paid, and were usually the first to be paid, as well; local soldiers, loyal or otherwise, wouldn't ruin their own homeland, but foreign mercenaries would pillage without a care.
Actually, it is. The attackers were spotted at the tree line where they exited the forest. The light sources are being placed 10m away from where Hubert was standing, but the last panel in that chapter shows that the tree line was at least 30m away and that the approach of the group he spotted was at such an angle that they had to have come out at least twice as far (60m).
In other words, they were at least 50m away from the nearest light source while Hubert was 10 and were thus getting only 1/25th as much light as Hubert—light that will be fouling up his night vision. And that's without accounting for extra shadows from the trees and scrub.
Sorry, but your distance measurements are all just conjecture, especially when we can look at page 13 and 14 of ch. 34 and see how completely wonky the distances that each of the servants placing the light boxes are being drawn at compared to the result; it's no good to use as a basis of measurement. The one placing one for the people to the rear is just a few feet away from the one closest in shot, while the one in the far right front is little smaller than the prince himself, yet is positioned like he's meant to be next to the cabin.
The very next page shows the cabins with the barrier boxes and five people standing nearby, all approx. 90 pixels tall from the same bird's eye angle, so there's no or little perspective skewing going on here; we can get a decent-ish (since it's still at the mercy of the artist drawing proportionately) comparison here. The distance from center cabin to the right is 340 pixels, which would translate to 3.7 times the height of an average man, and since this is a Japanese manga, the average would be 5'7.2", or 1.70688 meters; that means there's about 6.315456 meters between center and right cabins; from the left cabin to the center is 267~270 (lets round to 270 for ease of math) which would be 5.12064 meters. All told, they could at most be 10 meters away from each other if placed equidistant from the cabin in general, but nowhere near 10m away from the cabin itself, as you were suggesting. Furthermore, from page 24, we can also see that the cabin isn't that far from the forest line, given that from that position we can't even see the cabin to the right (from the bird's eye view looking at the doors) of the one that they are at (we might if the textbox wasn't covering that area, but there aren't any lines going beyond it, so either the artist didn't draw it at all, or it's further to the right, from this perspective). At most they're like 10-15 meters away from the treeline, not 50.
The lights also seem to be something like a faerie fire light that illuminates a wide area softly (and would make sense for long-lasting magic lights, since a bright light would require a larger stone for more charge to last the timeframe, and more magic is not something they have in plentiful supply in Wester, as has been set up in the story earlier), so I don't think that the light is affecting his night vision as much as you're suggesting, either; it's not bright enough to give away faces, at least, or else the leader would have been concerned about that when they attacked, after all. Furthermore, when the leader is standing by the doorway looking inside, if the lights were bright enough to ruin night vision, as you suggest, he wouldn't remark that it's too dark inside, given the windows (we can see two blocked off on the front right on pages 3 and 4 of ch. 35, and we can clearly see one through the doorway on page 29) and the open doorway, as well.
Hollows also create a crackly static noise when they're around, so hearing it be pushed to the side by the mercenaries seems to be more what he's detecting, rather than seeing it; it changes from a "vu...." to a "vun...." sound, so it seems to be a detectable change in quality. In ch. 35 on page 10, we further get confirmation that it's not visual recognition of the hollows but audible recognition, as they detect the hollows are being repelled by the sound they make against the barrier, long before they see the rag dragons coming in. It's also not until they leave the cover of the forest and enter the clearing that the Prince notes that they're here and confirms that they have a barrier box (which means he couldn't see), and then the guard on his left notes they're coming from the left too. Since it's the same bushes we saw on page 24, just seen from the front, we can definitively note that they aren't being seen all that far off at all, either.
Compare to Lia's escape:
It was almost dawn before she escaped. In that time, the kidnappers decide to and do signal to withdraw, Lia takes careful stock of the situation and environment and plans an escape, she backs out slowly, is noticed and then runs, starts making misleading grass movements to draw the kidnappers toward the cabins, then hides and starts sneak-crawling in the opposite direction.
At this point we get the last good shot of her and the kidnappers, putting her only 3m away but needing to pass by them. They notice they are having trouble and Lia notes that they are searching only toward the cabin, but looking at an angle that shows that they are not that far behind her. Then she plans the future with a diagram (unfortunately useless for estimating distance[*]). Then the hollows appear and at least five gather around floating over her. We get a shot of the kidnappers looking like it's from her POV (camera is low in the grass) where it appears to be maybe 15m away from the camera's POV.
Then the recalled attackers arrive, are asked if they saw Lia, and the sun rises.
OK, if Lia is 15m away, she is 4x closer (meaning 16x in visual area) in equal light to the kidnappers than Hubert was at a 25x light disadvantage. Hubert also spotted one lone adult-sized hollow while Lia is surrounded by 5. That is a 2000x advantage for the kidnappers, but lets say 1000x since the hollows might overlap from the kidnapper's POV and thus take less area. In order for it to be as hard to spot the hollows for the kidnappers as it was for Hubert, Lia needs to be 15m*√1000 = 475m away.
So if we assume that the shot that I got 15m from was not from Lia's POV, she and the kidnappers still had to separate from when they were within 3m of each other and needing to pass by, with both moving slowly (Lia crawling slowly to avoid disturbing plants and the kidnappers moving slowly to spot a toddler in tall grass under somewhat poor lighting conditions), in the time between almost dawn and dawn and only part of that. It's not reasonable that they could have gotten half a km away from each other in that time.
Time is difficult to measure in comics, but as noted above, there is more direct evidence of the distances involved.
She backs out slowly, and page 8 shows she's already a significant distance away from them compared to how she was right behind them on page 7, before she makes the rustling and they turn to notice her, at which point she gets up, barriers up, and dashes back into the tall grass,
where they lose sight of her entirely. Page 11 shows that they think she's heading to the seaside side of the clearing, and then turn around and head back towards the cabins in page 12, thinking, due to the rustling she did deliberately, that she's trying to get back to them through the shortest route possible, looping around them in the grass and heading straight back.
She's looking over her shoulder at them turned away, notes that they're all paying attention in the direction of the cabins, too, before the hollows even appear, let alone start crowding around her. While the diagram on page 12 is not to scale, it is still approximate, in that the kidnappers are at the northern edge of the glade through the section of forest that separates it from the cabins, while Lia is at the south-eastern edge of the glade, and the hollows don't appear until she's right at the forest's edge, as well. Again, you are weirdly estimating distances. Continuing on, the squad that was attacking the front and had been called by whistle to retreat arrives shortly after the hollows start gathering, just as the sun cracks the horizon, and they then notice immediately that the hunters are hot on their trail; no one really had any time to pay attention to something happening in the shadows of the forest at the far end of the glade that still wasn't lit even by the crack of dawn (given that the leader had to look over his shoulder to see the light peeking over the mountain peak).
As for it being "from Lia's POV", as you suggest, while I couldn't have confirmed or denied this previously too conclusively (other than the LOS not being in line with her head being so low to the ground), ch. 63.2 does in fact confirm that Lia could not see what they were doing at all thanks to the tall grass, and can only faintly hear the hunters yelling at the northern edge of the clearing, so it is a lot more distance than you have been thinking.
The hollows interaction with the world isn't relevant. They themselves cluster around victims; it's that behavior that itself points out Lia's presence.
Yes, it is, as they do not cause any further rustling and with their forms passing through solid matter, they become far less visible at a distance, beyond the fuzzy fading they have beyond close distances, as previously noted, or their translucency. If they had looked back and noticed the telltale signs of hollows, it would have clued them in, but it would not be an easy feat to do so, is the point.
I don't think they project light, either; setting up lights to see them at night would be counterproductive if they did.
I will take back one thing I said: It does matter if they project light, because if they did, it would wipe out the disadvantage that Hubert was at due to the lighting drop-off. So them glowing would actually weaken my argument.
The only way that they could see them as you suggested at those distances would have been if the hollows glowed, so I don't see how you think that them glowing would weaken your argument and them not glowing would strengthen it.