This is really the crux of it for me. Peer to peer is no good because it requires both parties to be online. The best I've been able to come up with is multiple

This is really the crux of it for me. Peer to peer is no good because it requires both parties to be online. The best I've been able to come up with is multiple servers, which work a bit like command and control servers for malware. Sync from client to server. Server replicates to other known servers. When a server is threatened, shut it down. Update server list on remaining servers to let clients know a server is gone. Add a new server and similarly let clients know. This requires at least 3 servers to be online at any time. It also assumes client-side key gen and encryption. There are a few more subtleties I'm building in, but that's my thinking so far.