We need more details on 6. This is the hard part, like you swap connection from A to B, but if B is not synced properly and you write to it then you start having diff between the two and there is no way back.

Like B is slightly out of date ( replication wise ) the service modify something, then A comes with change that modify the same data that you just wrote.

How do you ensure that B is up to date without stopping write to A ( no downtime ).