It is pretty unlikely that the game's net code will be completely re-written from P2P to Client-server.

PvPGN is a Battle.net emulation (lobby and mathcmaking, along with hosting closed Diablo games on the server).  None of Blizzard games have ever been p2p, they are always client/server.

The way Battle.net works, for custom games the host is the server for the game.  For ladder games or closed diablo games, Battle.net is the host/server for the games.

No, Battle net is a matching AND game hosting service.  Closed Diablo, Ladder SC and War3 games are all hosted on the Bnet servers.

When you create a custom war3 game (like DoTA) you take the role of being the server.  All players connect to you and they don't connect to eachother (which would be p2p).  Besides the only reason for playing DoTA on a PvPGN server is to be able to play with a pirated game.  There is absolutely no other reason not to use the battle.net servers.

Demigod runs in a synchronized environment. This makes cheating hard and ensures that the state of the game is exactly the same on each machine. And the p2p solution is the fastest way for this kind of environment.

If it would be client-server then there would either be no certainty whether or not the client states are the same at each point in time or it would lag more if it was a synchronized client-server environment. And in the latter case the host would be able to cheat.