Legally yes. Practically, the people handling the FOIA request do not know about it and do not have access to it, so they will not look at it when responding to a FOIA request. Also practically, if you submit a FOIA request for operational details of military action, the response will be no, and every judge you stand before to challenge that no will side with the government.

In the same way that Americans speak English.

Sure, their language is mutual intelligible with English, but if an Englishman comes over here and asks for some chips, they’re going to get a bag of crisps. They’ll mess up verb conjunction on a bunch of collective nouns.

And bless the souls of my Australian mates who come here and call everyone a cunt.

Just public key cryptography. All your actual posts and comment history are already shared. What is missing is a way to authenticate yourself to anyone but your home server. If the protocol included every profile having a public key, you could then use that to authenticate to any server. And managing that private key is no more complicated than managing your private key in a block chain context.

Non public info like subscriptions is a bit more complicated, because there is an actual policy question of who you share it with. You would either need to make it publicly available, keep a copy yourself, or have your home instance give it to you/the other server at the time you want to migrate.