If everyone was spread out onto different instances, and communities were based all over the fediverse, the decisions of one instance would be less impactful.
Which is why identities and communities on Fediverse should be cryptography-based, and an "instance" should simply be a sort of a supernode, or a caching node.
In principal yes, but requiring people to handle private keys would be a nightmare! Imo what we can and should do is support for transferring accounts between instances, including posts and comments.
If the account itself is like a property/attribute of a post/comment, then I suppose it can be changed seamlessly. But i dont think it is designed to be that way.
Afaik right now you'd have to send an update for every post/comment individually, if it would even work. I think we need one simple ActivityPub message that simply means "this actor is now this other actor, and all its objects should be updated".