The 'Lighter' Side Of The Force
One hole in this theory. When Rey was dueling Kylo Ren, he was trying to kill her as well. Rey wasn't just trying to avenge Han and protect Finn, she was also trying to stay alive.
I thought that immediately. She was using fear and anger at that moment. An untrained force user just delving into their powers could easily stray to the dark side. I suspect more of this in 8
Actually when she was using her anger kyle was beating her. At one point when she is on the edge of a cliff and kylo is in her face she closes her eyes and then beats kylo ren. That tells me she had calmed herself and used the light side of the force to beat kylo. The first time she uses her powers and entered kylo's head I would agree that was a darker side
It is possible she may have used the dark side inadvertently over the years but in your examples during 7 it was when she calmed her mind that she was able to garner the strength she needed.
Wouldn't that be a twist that Rey goes and finds Luke to get him to train her and then she turns to the darkside then kylo ren returns to the light side(remember he he says he feels the pull to the light) to help luke defeat her?