Perhaps I need to watch the film a second time to give a more objective assessment.
A storm trooper spontaneously deciding to leave the First Order and join the Resistance is equivalent to a member of Hitler Youth deciding to leave Nazi Germany and fight for the Allies. It is conceivable, but his motivation should have been explained better.
Well, Finn was forced into the First Order, unlike the Hitler Youth which joined voluntarily (based on various reason relating to ideologies, propaganda, etc.). But, yes, I can agree that it seems somewhat hasty, but the film didn’t exactly have a lot of time to spare on his motivation, which btw we don’t really know. He might have bee doubtful of the First Order for a very log time.
Luke was only able to deflect the blasts of the seeker droid and blow up the Death Star as a result of Obi-Wan’s training. Even if Luke trained Rey as a child before Kylo Ren wiped her memory, she should not have been able to subconsciously remember how to perform a Jedi mind trick as her memory was wiped.
Well, to be devil’s advocate, all Rey did was a mind trick and a Force pull, she hardly did anything fancy like what we’ve seen other Jedi do throughout the films and EU. Even Luke managed to deflect the blasts from the “seeker droid” after no more than a few days after he had learned that the Force was a thing that existed. Also, Ben didn’t give him much more advice and instruction than “feel,” so what it actually takes to do these things are quite debatable. You might even claim that the smaller things rely entirely on will-power and the knowledge that it’s possible. This was after all pretty much all Luke had prior to meeting Yoda.
If Kylo Ren massacred the New Jedi Order, why was he unable to defeat Rey?
Well, first of all he was most likely assisted by the Knights of Ren, and secondly an emotionally compromised man with a large bullet hole (I seem to recall Wookiee bow-casters shooting actual metallic bolts) in his guts isn’t exactly going to be able to fight to his full abilities.