It’s one thing to speculate that Luke has turned to the Dark Side by episode 7. It’s another to speculate that he turned to the Dark Side by the end of episode 6. In the scheme of the story arc of the entire first 6 episodes, it makes ZERO sense for Luke to suddenly shift over to the DS. And I don’t mean from the assembled evidence it makes zero sense, I mean there is absolutely no reason for him to do so. (Not to mention that him doing so would invalidate the prophecy of the chosen one that “would bring balance to the force” that Vader finally fulfilled once he tossed the Emperor like a bag full of Pikachus.)
Anakin was tempted by the Dark Side because he was arrogant, felt unfairly restricted by the Jedi council, and wanted a way to save his darling Amidala. Luke would be tempted by the Dark Side because… uh… because why exactly? NO REASON.
The prequels may have been shoddy movies but at least they understood character motivation and development, even if those aspects were clunky and poorly handled (seriously, we go from angst-ridden Anakin to Dark Vader in like 10 minutes of screen time?) If the series is suddenly just going to have characters go all topsy turvy, then it seems the filmmakers may have taken too many notes from “Rubber”. However, based on Han’s apparently personality reversal as demonstrated in the trailers I actually wouldn’t really be surprised if that were the case.
And, no… that’s not a “plot hole”.