I agree that the Rey as a Force creation by Palpatine is the closest we’ve gotten to solving this puzzle. It is tantalizingly close to working for me, and solves a lot of other mysteries as well.
But there still exists that fundamental incongruity with TLJ.
Stepping back a bit, I just had a thought about TROS that seems obvious now but hadn’t considered before: Why do we assume that Palpatine’s children and grandchildren would be strong in the Force?
Granted the Skywalker’s power in the Force seems to be preserved through lineage, but that doesn’t seem to be true for anyone else. Well, perhaps Yoda’s offspring, if that is indeed his offspring, but that just raises the question of why the Jedi wouldn’t raise families if it’s likely their children would inherit their power. I think it’s better than even odds that any spawn of Palpatine would have no power in the Force. In fact, why would Palpatine have offspring if they could potentially become rivals?
I realize that this is somewhat tangential to the current discussion since we’re working off of a different theory now, but it may stand examining if we are to figure out just what sort of Force user Rey can be. By the methods in use by the Jedi Order for over a thousand generations, a powerful Jedi coming from a random family would be the norm, not the exception.
The “fundamental incongruity” with TLJ can still be solved without changing Rey in TROS. Finn is a nobody who now inspires an entire stormtrooper insurrection (Ascendant v4). I’m sure we haven’t seen the last of broom boy, either. There will surely be tons of previously unknowns in the next movie who we’ll get to dig our teeth into. But the task of defeating the Emperor has always been something that somebody equally as powerful as him needs to do.
I’m not sure what exactly you’re comparing the Skywalker lineage to. Because we’ve only really touched on the Skywalker saga, there aren’t many other examples of children to Force-sensitives. The Jedi aren’t supposed to have attachments, either, so having children is a big no-no. But I see no reason not to believe that in at least 90% of cases Force-sensitivity is passed from parent to child. It probably functions like the magical gene in Harry Potter. Yeah, there are some squibs every now and then, but most of the time they get the same powers. Pretty sure Cal Kestis will be training that girl of his Force-sensitive friend turned evil, for example. And occasionally somebody magical is born to a muggle (normal person). But they’re never the strongest.
Palpatine would only create potential “heirs” because he wants the strongest to survive. I doubt he’s comfortable with the idea of creating his own offspring, because yeah, he’d probably see that as a threat to himself. He’d probably see that as a weakness on his part. But having others do the dirty work of raising the children and then waiting to see who strikes out with the dark side? Yeah, that sounds like Palpatine.
Oh, and the reason we “assume” they’re this strong is because it’s directly stated in the prequels and all current canon media. Yoda literally stops Obi-Wan from going to Palps because he knows how outmatched he’d be, and we know Anakin at full power is even stronger than Yoda. As Qui-Gon says in TPM, no Jedi in their recorded history has ever had as much potential as Anakin.