Steering this back to the topic of Thomas, the way he was talking about having kids, his family history, and how he feels about his demon, I don't see him keeping the vampire side of him. And then add in the trouble he has with Svartheim.
First off, (now I'm going with the idea that the baby is real and it's a boy) we know that Thomas is going to be a father. He is not needed anymore to further the Raith bloodline. The boy will take up that burden.
Second reason deals with his feelings about his demon. We know that he doesn't want to be who he is. He feels bad about taking life from people. He had been fighting the demon for a long time.
One of the only ways Thomas gets out of this is to become the WK. He takes up the mantle which is like a side shift in power, all the strength with none of the feeding, just obeying orders. He has Mab as a backer to get out of the trouble that he's in for the attempted murder. He still has standing with the WCourt because Lara wouldn't pass up the opportunity to have a tie in with the WiCourt, especially since the marriage won't last if it actually goes through. Come on, we all know Harry, things don't last with him around. And he would be free of the demon that has been plaguing him most of his life.
I don't think there's an evolving of the demon. It knows what it knows and hasn't changed, even when Thomas went on his hunger strike. Lara's comments in PT and BG kinda confirm it.