I really like your theories and i was kind of thinking the same.
But I did not think of your No. 4 and I absolutely love it (๑•ㅂ•)و✧
I wish I have your creativity. Theory no. 4 is my absolute fave. (≧∀≦)
Except theory 1 is exactly why I think the first heroes were dickheads. Their actions lead to precisely the situation that would NOT 'protect' her as they claimed they were doimg. They don't seem to have the intention to rub two brain cells together. Because the reason they would most likely leave her behind is because of something the DK said but they can't think of why he would say these things in the first place? That maybe there was a reason he did all these things where even a supposedly 'mere commoner' gets involved in the schemes? And that that would lead to trouble in the future?
1. The Hero has had an actual reason to abandon her each time that has to do with the demon king (aka her staying in the small town means that the demon king can't find her as long as he doesn't return) or something similar (basically, it was for reasons other than falling for someone else)
2. The hero is just kinda a dick and is just working through every possible relationship for each life and is basically treating it like it's a dating sim. The flashbacks are because she met the demon king in her 1st or second life (since she only remembers 3-5) and fell for him. He just hasn't had the power to track her down until now.
3. The hero insisted that she come with him on his quest during his 1st or 2nd life (maybe she had magic during one of her first reincarnations?) and the demon king met her then and fell in love with her, and has been stalking her ever since (w/ something like theory 1 happening)
4. (aka my favorite but least likely one) Theory 2 came into effect, but instead of just waiting and accidentally running into the demon king somehow, she got so fed up that she actually just walked across the goddamn continent until she met the demon king and proposed on the spot since, hey, we've both been fucked over by the hero, haven't we? and now he's been trying to keep her from getting her heart broken but hasn't had the power to prevent it in the last 3 reincarnations.