The intuitively obvious approach here is to have some kind of schema-guided or constraint-guided CRDT. That is, you get the guarantee that not only do you end up with valid JSON, you get the guarantee that f(that_json) == true. I imagine this is considerably easier said than done, but I'm curious if there's research (or results!) in the area.
Parts of the tree with “don’t merge children” might be ok, then you show the user a conflict dialog when this happens.
Or, inspired by your comment: run f for the tree, if false, search for the smallest subset where f is false and show the conflict dialog for that. The user
chooses “mine” or “theirs”.
