Must be honest and say I didn't read all the threads, so maybe it is allready in there, but here is my theory of Kian's mother..
What if Kian Alvane is one of the Draic Kin?
Then a bit off topic:
Plus, if Saga is April reborn it would make sense as she then later as Lady Alvane would just "return" to her home house. And also, the door in the living room that just has a wall behind it now, could be where April in TLJ come from even though we only see her arriving in the room with the fireplace.
Furthermore, it seems like Saga can shift when she places the drawings of Aprils story on the wall
Further thing off topic:
Have you considered Hanna to be the Red Kin reborn? i mean, her last name is roth (rot in German is red), her hair is red, and she is with the DRAGON-flies. Also supporting the idea that Queenie might be he Green Kin reborn making up for the past stuff. I mean, her eye shadow is green, the coat is green, queen is not so far away from green, and there is a dragon statue on her boat).. Then the two stark kins are still present
Your points about the Draic Kin being alive and Queenie/Hanna being them have been brought up before. I can imagine it's more likely than the whole Hanna/Anna/Saga/April idea that's been thrown about, but I guess we'll know soon enough.
My only queries would be the age disparity, and even the Green and Red working so closely together. Though having said that, it's possible their goals were never as far apart as we thought. Cortez is probably the most illusive of them all when it comes to motivations, though even the White's aims aren't that clear. I don't think the Blue really had any ambitions at all, but if he's going to be reborn, then perhaps that'll change.
Similarly, your ponderings about Kian and April being siblings is one that I've see around.
I don't think Kian saying ''By the Balance'' is anything more than a hint that he's become more and more a part of the resistance, and is using their terminology, similar to the fact I'm not religious, but say ''Oh my God'' when something shocks me. I could be wrong, or we both could be right and it's a nod to both.