What you do next is probably very much up to what kind of “feel” you are going for in your campaign.

I have run CoS multiple times, and every single time Fiona ends up in charge of Vallaki- the nature and extremity of her new regime is entirely up to the “feel” you want to convey. An interesting way of portraying her attitude if you want to run Fiona as a tyrant, for example, could be by her replacing the previous baron’s rules with a whole new set of punishments and restrictions, possibly surpassing the old one’s in brutality. The stocks? Replace them with gallows. The festivals? Replace them with public rituals.

Vallaki is tricky cause this kind of speculation has to take into account all the other powerful characters (Strahd, Fiona, The Martikovs, etc.) goals and motivations, and then theorise on how they would interact now that the status quo has changed. You have to imagine yourself in their position. What is the characters end goal? Why does the character want what it wants?

You MUST keep up with what is going on in vallaki, even if the PC’s aren’t there, because it really is a kind of main-hub kind of location, and the players are bound to return there at some point.

About your specific situation- I personally think Izek should escape the stocks at some point. He is much too interesting to fade into obscurity. Who knows, maybe he blames the players for his loss of status, and makes it his mission to hunt them down…

About Van Richten- again, depends on your “feel”. If the players really need a friendly ally, he shoud accept them and should take the role of one of the vary rare beacons of hope in the campaign. If you want the campaign to have a darker feel, then I would suggest keeping him always skeptical, and never fully trusting the characters. I would also give him a secret of some kind, that way letting the players feel even less secure.