I can't help but wonder if it's setting things up for the future, a mirror of what we're going to see; 'witness' and helpless Dean, sitting on the sidelines completely powerless and unable to help or save anyone...

That could definitely be going on! And if it means Sam gets to be the one who saves the day, I'm all for it. But if they make Cas the hero - if they let him undo everything Sam did and then "fix it" - I'm gonna be pissed.