Well... here's my thought process on Dean and Michael. I think Dean could have been persauded to accept Michael's offer if Castiel could guarantee that a) Sam would not be approached by Lucifer b) The angels would not let Lucifer out at all and c) that the end plan was not wiping everyone and bulking up heaven with humanity.

As Cas could not guarantee any of the above, Dean refused even though it put him in the catch 22 position of dooming humanity to the Ori or dooming them to Lucifer. Hence the despair! :)

But I am glad that you liked the SG reactions. McKay's I am saving for another day.