I don't think those translation notes on page 12 correspond to the translated text. Lower mana amplification efficiency means the effect will be weaker, while a higher efficiency means a stronger effect. This means that a longer incantation gives you better bang for your buck, probably because you have more time and directions to control the flow of mana, or something like that.
It's expanded with the left bubble in that panel, since he's saying that with a low efficiency, meaning the mana you put in doesn't result in a lot of power, you can still pour in a lot of mana to get a more powerful spell.
I don't think trying to recruit him would be harmful as long as they do so politely and without force. From their perspective, anyway.
Sea serpents aren't vicious and aggressive? At least in fiction they tend to be very temperamental.
I can understand the caution in confirming the kill. It's a wild story.
Why would they break off the fang? If they have the corpse, they can cut it out much more cleanly.
I'd expect the skin to be sold as well. Seems like it's rather tough, which is usable. The meat may or may not be edible.
That is a warning well worth the trouble it can stir up. The magic stone was the entire reason he went there in the first place. Though any guild that steals something that valuable will probably not survive anyway. Other than punishments, no one would trust them after that.
But considering their earlier treatment, I probably wouldn't accept that. Unless maybe if they paid extra.
Why want to keep the stone if you don't acknowledge the subjecation. Especially from a guy with a very worrying track record and nickname.
I'd probably just make that a condition: They can borrow it for a while if they give him the reward for the subjugation.