About the rewrite, I too was kind of annoyed by the summary. Especifically the "My solution? Gender-flip it!", because you changed so much more than that. Like:
1) The other heroes are actual jerks now.
2) The protagonist is getting harassed by the townsfolk now, or at least the threat of it hangs more heavily on her than in canon.
3) The motivations for buying the slave are changed to be more sympathetic. So is her behavior afterwards, in the long and short term.
4) The slave starts fighting for her willingly.
None of that derives necessarily, or at all, from just changing what Naofumi has in his pants. So when you summarize it as "The big change is that now he's a girl", it ends up sounding like you're coming in with the assumption that 1) Women are inherently better people, so you don't even need to actually
say that girl!Nao is more moral than her boy counterpart, that's just obvious 2) Women are inherently persecuted, always, everywhere, to the point where all five alternate universes in the story have misogynistic cultures. And in at least three of those universes, it's pervasive enough that three confused teenagers, inmediately after being taken to a fantasy land and suddenly being charged with a responsibility that could very well mean their deaths, chose to instead focus on how totally ridiculous it is that they also kidnapped a woman to fight; while a fourth man, the prince, secretly agrees with them but simply decides not to say it right in her face so he can more effectively get rid of her later.
In short, imagine somebody proposed the same extensive character changes that you did, but summed up the main thrust of the AU as "Naofumi is blonde now!". It's entirely the wrong focus.
Another odd thing about this AU is who, exactly, is getting genderbent. Nao, because that's the central premise. Malty, because the plot demands there be romantic overtones between her and Nao, even if it's all fake in the end. And Ralph, because...? Not everyone is getting genderflipped, after all; the non-shield heroes prove that. One must assume, then, that there's an specific reason for each change; thus there must be something boy!Ralph can do with girl!Nao that girl!Ralph wouldn't be able to. And Morton's Fork suggest that it's the same thing that only girl!Ralph can do with boy!Nao: provide romantic tension. After all a parent-child relationship can have just about any mix of genders, but romantic relationships are almost always one male and one female.*
I'm not doubting you about how you want their relationship to be, I'm sure that if you actually wrote the story you would write it romantically. I'm sure that this is just thoughtlessness, but it's precisely by uncritically copying the original work that you end up sending similar implications to it.
*I realize this excludes gay people, and I hope I haven't offended anybody by it. But in terms of tropes and media, it's pretty much true that no couple is gay "by default", as opposed to being practically the whole point of the relationship. This what makes Ralph's change interesting: in what contexts is it important that one participant in the relationship is a girl and the other a boy? Keeping this in mind also helps with making sense of Malty's genderflip.
Edit: To be clear, this is mainly a response to
@Jakinbandw's request for criticism. This was more obvious when there were less posts between our messages, oops.
