"Small" isn't exactly what I would use to describe shoehorning a 100% 6th Gen mechanic into an otherwise 100% faithful (right down to the black & white 8-bit text boxes) adaptation. Either be consistent and fully modernize Gen 1's story, or leave all of that stuff out. No cherry-picking.
Out of the major NPCs, only Lorelei, Bruno, and Agatha lacked official art (and technically, even they still had official art if you counted the Stadium mugshots, which were based on the game designs). Seeing as their characters barely existed in Origins to begin with, their lack of official art wasn't really a big deal.
If a 100% accurate RB adaptation is what you want, then, yes, you DO have to "take everything from the originals." You say that including the Sevii Islands would've "ruined" people's nostalgia, well, didn't the character designs already do a good enough job of that? Whenever I played Gen 1, Red always looked like this, not this. I did not grow up with FRLG, so elements from those games give me zero nostalgia. I know a lot of older fans also feel the same way.
Well, those people are wrong for calling that game canon, too. It's a great HEADCANON, since the games never established anything else otherwise, but no, it's not explicit canon.