My theory has been and continues to be that Kylo/Ben "falls" to the Light side in VIII, while something else causes Rey to go dark, and IX revolves around Ben trying to bring her back. Considering that Kylo's principle struggle is fighting to keep with the Dark Side, him going to the Light seems like an eventuality to me, so it's really a matter of how and when in my opinion. Rey has some subtleties in her performance and some specific aspects of her arch that make me think potential Dark Side user. Her rage at the end of the duel, her rapid and unprecedented accumulation of powers.
I definitely agree with "Oh shit,
her" reading of Luke's face. My thoughts are that Luke sensed she had far too much potential, and with Kylo he was terrified she would turn and would become a far worse power for the Dark Side. So I think he dumped her off hoping she'd never be found by anyone, in a bit of a fucked up but somewhat understandable exile. He'd rather her never learn the Force and be kept away from it all than risk the chance of her falling.
Unrelated, but a totally random theory I have that will probably never be touched upon: Luke invented the "Force freeze". The way I see it, it's a more defensive tool in inception (even if clearly it can be used offensively), being able to stop blaster bolts (and presumably many other things), and freeze enemies in place without physically harming them directly. It
almost has the same utility as Vader's favorite use of the Force, choking the shit out of people (immobilizing/neutralizing a foe), but requires far more power to do far less direct harm. Seems like something an extremely powerful Light side user would come up with.
I think he used the dark side to beat Vader in ROTJ but didn't fully embrace it and could control himself. He was just overconfident in ESB.
Yeah, I'd say that's pretty damn heavily implied. Once Vader wheeled out the idea of turning Leia he flat-out lost it and was fighting purely on total rage. It wasn't until he paused after neutralizing Vader and Palpatine starting cackling in glee that Luke reconsidered, recomposed himself, and rejected the Dark Side absolutely.