[quote=@Achronum]
[@Andreyich] I guess my concern about the age is the heart of the story -

At the end of the day, ATLA is a coming of age story where characters who have experienced things one way are thrown into situations that force them to look themselves in the mirror and make hard choices, growing as they do into the people they are at the end. It feels like this older character has already gone on this journey and come out, unfortunately, the worst for wear as a result. 

Talk to me more about how he'll grow and what your end goal is for his character arc as he embarks on this new chair in his life. 
[/quote]
I suppose, in that I have somewhat mistook the  RP's intent for while in the setting I didn't think we'd follow quite the team avatar dynamics (especially with the minimum character age being 18 xD)

My answer then, I think I would make twofold. 

His own journey begins as a jaded person unable to trust, because he feels that nobody in the world matches his own standards of being duty-oriented, loyal, truthful, etc. he will have to come to terms with the fact that people aren't evil just because they are not flawless exemplars of these things. At the same time, fighting against former comrades, he will be conflicted on if he is doing what is best since he still believes in a lot of the Earth Kingdom's supposed ideals. Moreover, he is now a traitor, doesn't that make him a hypocrite when he's all about duty and honour? In a sentence, his journey will come to be about realizing it is okay to believe in something, even if you can't live up to it. That he must tolerate imperfections both in himself, and in others. On a similar but not identical note, his journey will be about learning to let go. From family that is now possibly happier without him (and would be endangered if he visited them as a rebel, and his wife probably would have remarried as instructed by Temujin to further safeguard their children), to slights real or perceived from other characters (as well as getting over the fact many of them would have been enemy combatants just a few years ago), to possibly forgiving some of the people that helped bring his ruination as the party or he personally captures them, Temujin must let go. Just get over things, leave them in the past. Maybe also somewhat of a Zuko-esque journey in convincing everybody (and himself) that despite the fact he probably burned villages that their friends once lived in, he's a decent guy beneath it all. 

The second part of the answer is that perhaps he can be the one to help facilitate the growth of some of the other characters as he gets closer to them, using age and wisdom or simply being there for them when they suffer, not wanting them to go through pain that he has gone through alone. A more recurring version of Iroh, Paku, Guru Patek, Jong Jong, etc. Although perhaps you were wanting to leave that entirely to NPCs/GMPCs. 

P.S. I edited the sheet to account for your first two points.