Due to how the fediverse works if users can be given a did so can a community. It would only migrate for services that support did but wouldn't be all to different.
DID already exist they are a Decentralised IDentiy (DID) it is a keypair and user data so usernames, profile, bio, and a list of accounts across different instance that allows associating post comments likes etc.
No thats the whole point of a DID. Its an existing standard that has been established to manage decentralised identity. Their exists multiple ways to handle it so a did is did:source:id where the source can be many different things blue-sky uses a group of trusted identity server, but u can use a selhosted file, the blocckchain all sorts of things. Hell u could even use bluesky u could have the same login for ATproto (bluesky) and activpub (lemmy)
Well peertube is struggling already so thats a valid question. Idk if they using peertube style bitorrent video delivery that might help offload the cost a little.
Yeah that's critical without it everything would break