Interesting theory. Not sure I buy Luke turning to the dark side before our very eyes; that requires a subtle interpretation of an action / adventure movie. However it’s completely plausible that the events after Jedi, continually push Luke toward the dark side. With Vader and the Emperor gone, there’s a power vacuum. Luke is the last jedi. How does he handle his new responsibility? Without Yoda or Obi Wan to guide him, what choices does he make? The path to the darkside is a slippery one, and Luke is already halfway there in ROTJ.
What’s great about this theory though is that it resolves the fundamental crux of Empire. Luke (as you pointed out) was given a choice: Let his friends die, or destroy everything they’d fought for. Luke chose to save his friends. But the consequences of this decision never play out. His friends survived, and they overthrew the Empire. He has his cake and eats it too, making his decision in Empire irrelevant.
I think that The Force Awakens finally plays out the natural consequences of Empire’s big choice. 30 years later, we see the real repercussions of Empire. The Emperor was destroyed, but his regime lives on. Luke has fallen to the dark side, and is running the New Order (or facilitating darkness behind the scenes).
Also (and I don’t have any proof for this, other than the movie’s title) I suspect there’s been a lack of force users in the last 30 years. In the time of FA, Luke is the only Jedi / Sith, and he’s been waiting for a new force user to come along who can carry on his legacy. This means Kylo Ren IS NOT a force user: He’s just a badass fighter with a lightsaber. While the trailers don’t prove this, they also don’t disprove it: We have haven’t seen him doing anything force-tastic.
One of the two main characters (I suspect it’s Rey) is the new force user, our generation’s Luke Skywalker. Luke has been waiting for his new protege to show up, and this is the driving force behind our plot. Once Luke realizes what’s up, he’ll pursue Rey to the ends of the earth until she agrees to join him in his sinister plans.