Yseult's orders will be to watch, strategize, and report, but if a good opportunity to test the waters presents itself, he can take it at his discretion, as long as the risks (of discovery, death, or damaging future mission prospects) are minimal.

Desi will find that the highest-ranking and most influential leaders of the Minrathous Ambassadoria never leave the subterranean levels of the complex in order to avoid losing their caste. Getting at them directly as a human (even a short one) will be extremely difficult and frankly not worth it at this stage, as there are plenty of lower-ranking officials who do venture out and about. There will be plenty who are boring either in personality or role, so feel free to make up whatever surveillance shenanigans and minor foibles you please. The most promising candidates for potential further action will be:

Hardis Gatterhorn, a petty bureaucrat in the sort of mid-tier paper-pushing approvals-type role that involves two of her favorite things: doing relatively little actual work and having the power to make everyone else's lives harder by holding up theirs. She is taking bribes from at least one Venatori agent (unclear for what), but she's also taking bribes from several other people. She doesn't have any sob story to fund, she just loves having money to spend.

Lemmit Nista, probably the most promising target in terms of access as he is a personal secretary to Lord Dozon Lunksorn, the second vice-consul of the Ambassadoria (confusingly, third or fourth in line overall). Lemmit is (surprisingly, given Orzammar prejudices) a surface dwarf, born and raised in Minrathous and still living at home with his family. His youngest brother is also a guard at the Ambassadoria, a post said brother has been using to sneak out the odd sack of lyrium vials to sell through his gang pals. Lemmit doesn't seem to know about this, but he probably got his brother the job.

Kallo Forbis, an academic focused on studying lyrium and liaison supporting Orzammar's contract with the Minrathous Circle. Last year his wife, also a scholar, was killed in a mysterious laboratory accident of some kind. He has become isolated since her death, developed a drinking problem and fallen into debt with some unsavory folks.

If you're dying for some other option, feel free to make somebody else up on a roughly similar level of risk v. reward.