Yeah, I'm glad it ended optimistically as well.
You know what I do love about this episode? What he says to Takatora at the end:
"But you know what? I want you to teach Mitchi - how to accept the past, how to find a new path and how to move forward."
"That's something not even I could do."
"I don't want you to give up on him. People can change. Even someone like me - I was able to become someone else. It's called transforming, Takatora."
"Kazuraba..."
"If you can't forgive the person you are now, then change into a new you."
That mirrors Kouta at the start of the series, who wanted to "Transform" as well.
It's been one whole year and we've seen how these characters have grown up and changed. If Takatora represents hope, and Kouta represents growth, then with that chat at the end, we can assume Mitchi can and will be redeemed. No matter what he has been through, what mistakes he has made, he can still change himself and grow up to be a different person. Maybe that's why Urobuchi finished writing with this episode, because symbolically and thematically, it brings the series to a full conclusion.
Gaim's themes of evolution, human nature and moral consequences may have fallen short of their intended goal, but I'd say the theme of growing up was fully realized. If you view Gaim as a show about that sole theme, I think it's safe to say it was brilliantly written.
In the end, the show really goes to Mitchi - How Mitchi wanted to be someone else but ended up making the wrong choices and faced the consequences for it. But in the end, even someone like Mitchi can change for the better.