Slipping that bit of archangel info to Dean was definitely moving into gray territory, IMHO.

Yeah, I think so. Cas only said it when he thought Dean was going to leave, and as a way of explaining himself. You could probably read that as Cas trying to manipulate Dean into staying and not turning against him, by giving him some kind of hope that Sam won't be killed/go dark side, but maybe it's just my stupid romantic heart, I think Cas is now finding he needs to explain himself to Dean. Not just because Dean demands it, but because it eases Castiel himself in some way.