The way I understand it is that they can relicense it and then publish it if they want, but the GPL would still fully apply to the previous versions.
The first question you cited seems to refer to any different organisation/individual making changes to the source code. And the second seems to refer to revoking the GPL for an already released version, which they would of course not be allowed to do.
This would make sense as ownership of the copyright would supersede a license.
Well I'd say most of them are federated together, or at least those with a good amount of users. In practice you don't really get islands other than I guess troll instances that everyone has blocked.
And AFAIK as long as an instance isn't blocked by yours (and vice versa to be useful), you can follow a person on that unfederated instance and it should just work and get federated.