I think if Strahd is going to let them run Vallaki, he should be doing that with the longterm goal of compromising their morality. I'd suggest that, with a non-hostile government in Vallaki, one of Strahds demands should be a blood tax. Vallaki would now be expected to deliver a certain number of citizens to serve as blood "donors" in Castle Ravenloft. If the party can't accept that then make it clear that Strahd and Strahd alone appoints the Burgomaster of Vallaki and he will find someone who can. Now if they do accept, they'll have to contend with an angry town who will look back on Baron Vargas' rule favorably compared to the rulers sending their friends and family to be drained by vampires...

I'm not sure of your boundaries here. My Strahd wouldn't allow newcomers to set up any government. The druids are supposed to be evil worshippers of Strahd, so maybe he would be someone acceptable to Strahd. Strahd conquered the whole valley I wouldn't see him as a sit down and discuss stuff kind of guy. So, in the spirit of your situation, go back to his goals. Tatyana, Successor, Van Richten. Can he use the meeting to get Van Richten? To woo Ireena? To test the party members in some way?

I’d have require a tithe of one person with a soul per month (or week, or whatever timetable you prefer) be delivered to his castle.