1. The same API, but according to
https://developers.google.com/cloud-messaging/android/android-migrate-fcm (all the way at the bottom), the endpoints have changed. Hence the firebase infrastructure may be in better shape if they just keep GCM running for legacy-users and don't spend much time on maintenance.
2. Of course fixing the issue would be the better way. But until the core of the issue is found, some workaround should be in place. And as long as the connection between your server and GCM is stable, there won't be any restarts. To reduce unnecessary restarts you could also delay the automatic restart to wait until three consecuitive messages failed to arrive.
And just out of curiosity: by server you refer to the process/application, not the whole machine, right? So there shouldn't be a long downtime.