I would squee like a very squeeful thing if Castiel got the order to pull Dean out of Hell by someone who claims to be God, but who really isn't.

That would be the one thing I would find really disappointing. Castiel is so creepy because he is a messenger of the Lord and had the power to raise Dean from the dead when no one, not even Sam, could do it. To say it isn't so would cheapen the whole thing.

As long as God doesn't appear as a character and remains and unseen presence instead, I have no problem with his messengers taking an active role. After all, the whole hell thing is based on the Christian version of hell. If they didn't want to go the way of medieval Christian lore, they should have stopped a long time ago.