Well, Damon did jump into bed with Elena within 24 hours after that break up so he was enjoying it some degree.
Maybe, but that's beside the point since I was talking about how each brother reacted when the other's relationship with Elena ended: they had the same reaction.
We haven't got a "I'm not not happy for you" from Damon, but he did push Elena towards Stefan more than once while she was making googly eyes at him (Damon), that should count for something.
But I think it's a matter of what the plot needs at any given time, really, it doesn't have anything to do with character growth. Stefan being now a fan of Elena's relationship with his brother after spending the first 4 seasons of the show telling her that Damon is the absolute worst is the same as Damon trying to get into Elena's pants one episode and telling her not to give up on Stefan the next one: they do what they need to for the plot to go from point A to point B, that's all. I'm pretty sure the moment the writers decide to resurrect the triangle, supportive Stefan will suddenly disappear.