1. I'm pretty sure the "she" is Yuko aka DOUBT, the girl with lavender hair from the upside down house that Kyoko was in love with. Futaba has been connected to her before because of the lavender (Futaba smells like lavender). It seems like Yuko had the ability to leave the house (getting groceries and books) by physically traveling between planes like Futaba does. I'm not sure exactly what Futaba's connection to her is, but it almost seems like Nodoka is suggesting that it's hereditary; is it possible that Futaba is her daughter? Maybe Yuko left the house permanently at some point, or maybe Futaba is unknowingly adopted.
That's quite a lot of extrapolation solely based off the fact that Futaba uses lavender-scented perfume. There are so many assumptions here that need to be all correct in order to have "Yuko aka DOUBT being this
she" make sense when we can't even confidently discern what the upside down house really is.
To streamline your claim (apologies if I sound impolitely sarcastic), that means Yuko can leave the upside down house for books and groceries
thus can also travel through planes, and got pregnant
on aisle 23 when she was outside, and gave birth to Futaba, and had her adopted by others without Futaba's knowing it, and Futaba chose lavender-scented perfume because of the hair of a woman who Futaba doesn't know that's her bio-mom, and definitely not because it simply smells good.
I get that you're trying to fill the blank between "Futaba traveled through planes for the first time" and "There is someone in the upside down that has lavender for hair and Futaba actually uses lavender-scented perfume" and explore a potential bridge, but to do so all of the things described above would have to be true to a certain extent, otherwise the chance of this
she being Yuko drops enormously.