Hi GMs

We are almost finished (read: we are currently in the adventure before the last one) the "Tales of the Demonlord"

My problem: after the party was able to catch one of the henchmen of Lucretia (I let them watch the party for almost the entire campaign) they now sided with her....so now the whole (and kinda stupid) "I'll kill everyone in the council until I get my artifact back" plothook doesn't work.

The party already has suspicion towards either the dwarf or the halfling in the council and one player recently got the hint that it must be the old halfling (which they kinda like as he is an old gramps)

So o don't know how to start the final adventure. One player also got voted into the council since he is a) now in good terms with the rich since he provides them with fancy scrolls to change Outfits and b) is now rich because the mentioned trades and contacts he has made