Dean.
They don't teach any of this history in Sunday school.
I can see why he'd agree to it in your world. Desperation will make people do all kinds of things. And if Lucifer isn't in Castiel here, he'll need another vessel, which I doubt Castiel would want. Maybe he's hoping he can keep Lucifer in check enough to buy us time to take care of him. The alternative is letting him loose, and that won't go well for any of us.