Would it be feasible for you to create, say, a "mini cellar" without negatively effecting the game's pacing?
It was mentioned in the Q&A for the Undead stream that you cannot edit/create maps on the fly. You can add creatures and do a lot of modifications to existing layouts but for purposes like that you'd have to stop, create the new zone, and re-enter the game.
You could also just create a bunch of small maps beforehand that you can load up as generic dungeon areas to explore, just replacing whatever creatures/items you need in them on the fly.
Is it possible to have everyone on the same map, but only allow certain PCs to interact with said map (to avoid having others just walking around screwing up said PC's scenes with random shenanigans that only players can dream up)?
Not directly, but you can apply status effects to the players that effectively lead to this. You also might be able to make disjoint areas of the map or areas that are in-escapable, and you can drag the people you don't want acting into those locations. Or just rely on people being good
