Alright I said I'd leave it at the previous, but I'll bite.
Everything in your second half is from when Natori first showed up in the world and was working off her perception of the characters as she knew them when it was all just a game.
Of course she'll fangirl over Diana The Protagonist and be intimidated by Lapis the Antagonist.
I maintain that her "adoration" for Diana is that of a fan and her idol, not someone developing romantic feelings. If anything, it's more like sisterly love than "I want to be your wife", at present.
She comes to have a deep connection with Lapis, however, as she learns how different Lapis is from her portrayal as the villain of the game, and it absolutely becomes at minimum much more romance coded at the point in the story we're at, especially compared to what Natori has toward Diana. Yes, Diana very much likes Natori. But it decidedly one-way in terms of the specific kind of "like".
I'm also working entirely off the manga and have not read the source material, so forgive me if I'm not pulling from events not yet portrayed when I make my point.
And I did say "has", not "had". So referencing "initial" parts from the beginning of the story like the characters haven't changed since that point feels disingenuous to me.
Beyond that, it's really whatever. I personally don't see a reason for a poly ending. I won't belittle you or try to take away your enjoyment of and desire for it, but we're not going to convince the other on this point I think, so I'll once again just say "whatever happens by the end, hopefully it's justified by the writing in-between", and wish you well.
