That chapter is somewhat wholesome but inspite of what Nanase did : it's all thanks to her mom actually caring about the kid doing all the cooking alone.
This is extremely reminiscent of the chapter where Yuni invited Nanase to a festival and her whole band ended up coming. Nanase, although she acknowledges that she is simply doesn't do enough (which is fairly basic), that is, spending time together. She literally abandoned Yuni to do the cooking chores instead of you know, inviting her to have a good time by the river or with her dogs ? Being a gracious host to a guest ?
Honestly, for people not minding Nanase's behaviour here : imagine your lover invited you to do something like a canoeing, suddenly they ditch you with their friends to go play by the river. Meanwhile, the instructor comes and grabs you : you're inflating those canoes, installing the boards, etc, you're carrying them to the river, with the only help of instructor (if you're wondering, that shiet can weigh), suddenly your lover comes up and says “oh why didn't you join us to play a bit ?”
What is your honest reaction to that ?
She is sweet probably (with that nickname Nappi) but not to her lover : as people already pointed it out, she let Yuni deal with the whole travel and lodging alone just to see her play and not even hang out after the matches.
Let's scale it up : imagine your lover asked you (quite forcefully) to go to another country for a whole month so that you can spectate whatever they're doing (a match, a conference, anything). But you'll be staying in different hotels, you'll have no direct contact with them, so no dates or visiting the country together for the evening or any free day. They ask you that a month before the departure.
On the other hand, you don't have a lot of savings and your job is minimal wages.
The normal answer to that is : “are you dumb ? unless you're going to include me in the group travel or something”. But because Yuni has a really strong idea on love and Nanase's wording, she felt like she had to, for her, and she effectively did.
Now imagine you actually went through all that trouble (depending on the country, a visa, the money to pay a hotel for a month, the plane ticket, etc) and your lover didn't feel grateful but indeed thought it was a given when you effectively went up and beyond for them.
Yuni needs to reveal all that to a no-nonsense friend (secret be damned) who'd be able to convince her to simply break up, help them both out that this relationship just ain't it.
As to the prologue : you clearly see that Nanase was getting flustered by the compliment Yuni gave to her haircut. To me, it certainly seems more like Nanase was the one who began to fall in love, because Yuni was doing something fairly basic, give a compliment when you see something worth it, gave her confidence ; something that her own friends didn't give. So Nanase isn't a pure volleyballhead, she has some sensitivity about her style, her appearance.
On the other hand, i think i remember in a chapter (the one where they're in a different city) that Yuni, who put up a lot of effort in make-up, hair, clothes, say something like “you're different”. If that's not a literal douche answer then, i don't know.
