I'm in the camp that thinks that Kylo saved Ren in that flashback scene.
Someone can correct me if I'm wrong, but I'm pretty sure that in order to fully go to the Dark Side, you need to kill family or friend. Kylo saved Rey, which was why he had a chance at "redemption" in episode 7 and killed Han to complete his turn to the Dark Side.
I'm also pretty sure Rey is Luke's daughter. There are just too, too many references for it not to be the case, and if it's not, it feels deliberately deceitful and not part of the narrative. Besides, did anyone mention that almost every time Rey does something cool, the theme music is the same as Luke's?
There will be big reveals in episode 8 for sure, but I don't think the family ties will be one of them. They didn't do much to keep up in suspense about Kylo's heritage, and I don't think they'll do it for very long with Rey. But considering that leaving the question unanswered in this movie means several million dollars more in ticket sales for repeat viewings to look for clues, then it's a smart move.