You've tried to CONVINCE me that that argument is false, and I disagree, and have given counter arguments as to why I disagree. I don't think what you're saying about her is enough to justify the Mary Sue label. Captain Marvel is a Mary Sue. Hermione for about, the first four books, is a Mary Sue. They're perfect, and can do no wrong. As well as have zero flaws that have ramifications on the story at large. Rey, does. Her whole training with Luke and going straight for the Darkside is an example of an entire SCENE dedicated to her flaws, where Luke has to effectively save her, and admonish her for her lack of control and idiocy.
As for the Luke and the Force issue. He didn't even know what the Force WAS until the conversation with Ben. Who dies literally HOURS later. And we have zero proof of him training to use the force between those two movies. It's done off screen, and is even actually stated that he does it. So, from a story telling standpoint, that's JUST as much of a Deus Ex Machina as Rey using the force in the Kylo Ren fight. She had just as much Force Knowledge from Leia as Luke got From Obi-Wan if we are going off of screen time.
Your argument was objectively wrong though. You can defend the writing all you want, but you keep saying she can't be a Mary Sue because she displays flaws which is false. Again, your idea of a Mary Sue is flawed. You keep pushing the notion that being perfect and flawless is all that characterizes a Mary Sue when Mary Sue herself wasn't perfect and the idea behind her was to highlight bad writing that creates unrealistic scenarios in service of the character, such as established characters not acting in character and being impressed and outdone by Mary Sue herself. Which describes Rey perfectly.
I can't speak for Captain Marvel or Hermione as my knowledge of them is limited, but being perfect and flawless is not what makes one a Mary Sue. Characters can be flawless and do no wrong and still not be considered a Mary Sue so long it is well written/believable within the context of the fictional world, which is done by establishing a solid background.
Rey and Luke are essentially the same character, which is part of the problem. They go through the same exact journey (which is lazy writing for Rey) but the outcomes and the way in which they get there is completely different.
From the start, we have a very clear understanding of who Luke is before he embarks on the hero's journey. He is given a solid background, motivation, and believable skills that he puts to the test throughout the film, yet he never demonstrates anything close to what Rey can do because he's still your typical everyman. Meanwhile, Rey has just about the same skill set as Luke by being a scavenger and has heard some stories so somehow that makes her very competent, independent, and powerful throughout her journey.
Cherry picking moments without any regard of context and comparing them doesn't make them the same and it only highlights how terribly uninspired the sequel trilogy is as it's merely a cheap imitation that fails to understand why those scenes worked and weren't just there to make Luke a "badass."
While Rey displayed need for training and was drawn to the darkside just like Luke, her selflessness and hyper competence diminishes her flaws whereas Luke's flaws led to direct failure. They're doing all the same things, except that Rey is doing it all with cheat codes which is very uninteresting and well, Mary Sue-like.
Luke had to grow before he could fight with a lightsaber, use the force to move objects, and Jedi mind trick. He's able to do these things throughout the span of three films and he still fails. He doesn't do it all on his own either. As you said, the problem with Rey was treating her background as a mystery. This was fine for the first film because we were all led to believe they had some sort of plan and knew where they were going and then The Last Jedi happened and threw a wrench in the non-existent trilogy plans. Like the asshole kid who purposely tries to simultaneously tank the last and next sentence in the chain story.
You can argue The Rise of Skywalker fixes her Mary Sue status, but for me, it really doesn't and it's too late. It's just terrible writing. If rumors are true about the Justice League-like production of the film, then it only further cements what a poorly thrown together trilogy this was and Rey is no exception as she's just female Luke but better in every single way. They completely undermined the purpose of the first 6 films for Rey. The actors are great and the characters are fine, they're all just incredibly wasted and feel like nothing more than new action figures. It's like a bad fan fiction.
, he lost the fight ( not to mention Rey knew a thing or two about fighting techniques so it was not like she was completely helpless).