Tiaral

We could, of course, pull a fast one. Send in a spy, capture and interrogate him, and then pretend we completed the quest to get the antidote. But that depends on whether Sharon will cooperate.

A perfectly viable option. Eliminate the witnesses, do everything as planned, and then negotiate with Sharon and hide him in a closet. If he's a reasonable person, he should see a good opportunity to trick the Fog People, who wanted to eliminate him. The obvious downside is that if the fake Sharon does something unexpected (a vial of poison hidden in his cheek?) and we don't get the information we need from him, the whole operation will fail.
I don't see any point in outright refusing to deal with the Fog People. That way, we'll lose on both fronts, both in terms of casualties within the group and in terms of our ability to outsmart the Fog People. And we won't be able to complete Sharon's quest either, right.
So, in my opinion, the options are to dance to the tune of the Fog People or pretend to dance to it. /

What's so unbelievable about that? They took or cultivated a naturally poisonous fruit and sprayed it with pesticides. For good measure, they added a secret cheese ingredient from the cook, which is difficult to identify blindly in a laboratory within a timeframe sufficient for a cure. If we abstract away from this damned medieval setting and imagine that the antidote is something more complex than some dietary supplement or plantain ground in grandma's mortar.

Judging by the fact that no dispels help, it's not just poison, but something more like a colony of fungi that gradually works. And it can't be dispelled because dispels remove the result of the fungi's activity, but the fungi themselves, as a living organism, cannot be dispelled. But that's just a hypothesis. /