Dark elf isn’t much of a threat since she’s very sick as a result of having learned 3 elements. She could fight but the strain would take years off her life and might kill her. Maybe they think she helped mc wipe out of the guild, but if so, the strain would have probably left her bedridden. plus, attacking one thieves guild means making an enemy out of every thieves guild, some of which have some descent fighters.
As for why they’re underestimating her, I do think few factors come into play. There is a very weird rivalry between scouts, thieves, and assassins (all 3 literally tell mc not to compare them to the other 2 at different points in the ln), so the thieves would’ve likely (consciously or not) downplayed the strength of the assassins and been willing to believe they fell into a trap as simple as mc blowing up their base. That’s a very assassin thing to do. The assassins also appraised mc when she first joined so the beggar could’ve known her stats from back then which are way lower than they are now.
1. That still does not mean that the knowledge of the dark elf and her standing in the guild AND the fact that she is leverage over Aria is known to the thieves guild. My point was that the most they would know about is the Elf and her relationship to Aria if anything and that alone is enough to be cautious because you were trying to make the point that Aria was known as the leverage over the Elf. My firm contention is that none of it is public-ish knowledge so it will not factor into their consideration at all.
2. IF we are to believe, as you're trying to state now that, they somehow not only are supposing that the elf is involved but also know about her powers AND limitations AND are able to extrapolate her current state then why would they not mention it in the conversation when offering protection to Aria. Highlighting a weakness of the other party in order to make your own strength seem greater is negotiation 101. Still this doesn’t really matter because it’s not in the manga or in the implications at all. We don’t know about how the thieves guild functions; we don’t have any indication of what their offensive force is like; we DO know that rumors were spread around after this event, but given their vagueness we don’t know to what degree the actual facts were shared.
3. If it’s about their status then AGAIN a phrase like “She’s just a single assassin; Don’t give her time to run; Don’t give her time to think; She can’t have any traps rush her quickly” All of these then would center around their, as I stated in the earlier post, overestimation of their own abilities and in such a case an underestimation of her skillset. NOT just the fact that she is a little girl, which by all accounts as I am saying should be very far from their mind considering any appraisal of the situation.
4. I don’t know how or why you think the assassins guild would actively share their appraisals of active killing members with essentially their doormen when they don’t share appraisals between said members so as to protect their methods of killing and capabilities.
You’re doing a lot of “writing for the Author”, where you kinda have this head canon of explanations and justification that aren’t supported by the text at all. It stretches the idea of “could” beyond it’s limits merely to MAKE some kind of resolution instead of piece together or uncover or construct some kind of resolution. Though this manga is great and has a ton of good writing, just call out when it’s bad writing.