I really can't see it as anything other than "Natsumi has crushed on Futaba since middle school due to her being the only person who didn't eventually leave her due to her attitude and personality (she is convinced her personality sucks, IIRC), but she gave up on that crush a long time ago but is also still holding onto it. +Yuuto is a stand-in for the role Futaba used to play, and I'm not convinced that she has any feelings for him outside of using him as a placeholder.
Sure, you can reference 196, but in 197 she spends the whole chapter talking about Futaba, with clear subtext that it wasn't just those panels where she was talking about her, and Yuuto even thinks to himself that she may like Futaba as more than just best friends.
Then we look back at chapter 121. Not much here, except that Natsumi has her fingernails painted yellow, like usual. Except one is painted green, like Futaba's hair. Her left. ring. finger.
Also the tags on the latest pixiv post are pretty indicative. "想いをひた隠しにする親友の話" Like, COME ON. My best attempt at a smooth translation of this is "A story about hiding your feelings from your best friend at all costs". There are other ones like 「早く告れば良い後輩の話」which I don't really have the expertise to translate properly (hayaku kokurebaii [or kokurebayoi, but probably the former], the 告れば良い gives me trouble. 告れば is the hypothetical form of a slang for "confess (love)", but ~ば良い can mean different things than just "if". Any native speakers would be great to give a couple possible translations of 早く告れば良い後輩.
I rest my case. Natsumi is a closeted bisexual (or lesbian since we find out that she never brought anyone back to her apartment, and also I honestly don't remember if they ever mention sex at any point, but I don't think they do) who is still holding onto a crush that she has on Futaba since middle school, and Yuuto is just a surrogate for Futaba.