Git is decentralized by design. It can support federation, it just happens that GitHub solved the UI, issues, PR so that even new comer can come in and do git stuff and track issues on the screen. But centralized it.

Federation would be closer to git, but not so decentralized that when one node goes offline you may not have any upstream to pull from, or not be able to find them.

Git doesn't solve availability. Federation may solve it, by staying closer to the decentralized philosophy. That's my read.

Not sure I understand, you're talking about mirroring git repo data between multiple different nodes? That seems unrelated to what's proposed in the OP--maybe you're seeing something I'm not?