At least in case of XMPP, the client doesn't need to be able to connect to other domains, so as long as you can connect to your own server outside of the censorship's reach (which could be accessible for c2s connections in a completely different way than for s2s), you should be fine.
Yes, but it's always possible to block IP (targeted attack). Federated with a big amount of small servers make it hard to automate. You can block several hosts but the rest of the network would work fine. And because of how federation in XMPP works you just need a one client to server connection to reach the entire network.
