I was reading the whole set of theories thinking the same thing that there are likely a lot of descendents of Revan and Bastila, so Theron being the end of the line is not likely.
That said I have spent the last few hours reading from the OP to this point. The OP was very well thought out as were most of the others posted. Agree or disagree, it has all been a good read and I love the various thoughts and theories you have come up with.
I am also of the belief that Theron is helping the alliance and not hurting them. He is an accurate long ranged shooter and would not have missed shooting Lana 3 meters in front of him, or the commander. He created a way for them to get off the train and his halo call to taunt them was his way of checking to ensure they made it off the train safely.
But I will say this -
When he is reunited with my Jedi Knight, she may have been all sweet in telling him she still loves him and to come home, but after she has had time to think about it, he will have some splainin to do and it better be good cause she'll be standing there with her saber and will not hesitate to start eliminating body parts.