I'm super confused on the demon blood idea as well. destina made the connection that that might have been why Ruby had to have Sam all juiced up on demon blood at the time, not to kill Lilith but to protect him when Lucifer took his vessel like what was supposed to happen that night. But it makes little sense to me, that he'd have to drink demon blood to do this. It'd feel counter intuitive on the whole redemption front. Why the small bit of demon blood as a baby was needed then anyway? Idk. And then why would Dean be able to contain Michael without it? Unless they purposely brought him back more intact already?

Of course, there's always the fandom idea of Sam drinking angel blood instead too. Who knows.

I think the everyone underestimating Sam comes back to The Curious Case of Dean Winchester. Exact same scenario. Sam being underestimated and winning.
Ah, I was reminded about that last night. When Sam had his 'but I'm all we've got' line. Pushing out the cobwebs, but what'd he do there? Kinda used his weakness over the idea of Dean dying and turning it around on his opponent, right? Seems applicable now.

If there's God interference in the finale (and there was in the premiere so, knowing Kripke...) I'd put money on Cas being safe. No way God's gonna let his most faithful angel die. He didn't before anyway. :)