My question is, do you think Cas knows about Meg's death, and if he does, how would he react?
No... No I don't believe that Cas does know that Meg is dead. You see, Cas only has three people in the world that he cares about, and those three people are Dean, Sam, and Meg. And for Cas to lose one of only three people... Cas just loses everybody.
I can't catch a break... Wow. That was a depressing question. Thanks for bringing the mood down.