The Mona situation feels like it's closely mirroring Kokoro pressuring Kurumi in chapter 7. They're even sitting at the same table from back then (I wonder if it'll flashback to that time actually). And of course, Mona wanting to keep all of this a secret from Naoi is a giant red flag, especially after Naoi told Kurumi only last chapter that she knows something's up with Mona and wants to keep an eye on the situation. I'm hoping that this is just the final narrative test to show Kurumi's character development before Naoi accepts her confession and before Kurumi finally faces her mother after all of this time.
Naoi's thoughts this chapter show that this would be the worst time for a perceived betrayal, because she's already developed such a negative view about love and affection, that any more piling onto it would almost certainly result in her rejecting Kurumi's confession for good. I think the story is heading toward her needing to perceive the positives of love, like loyalty and putting the other persons well-being over their own desires. Mona's interest in Kurumi is likely fake or at least largely exaggerated due to wanting to use Kurumi to help solve whatever is going on at home. So, the arc seems to be about Kurumi not getting misled by fake love versus real love, while Naoi's struggling to put her faith into love.
Considering that Kokoro was on the color cover of the volume, I'm guessing her role will be during the school festival, which is what I'm assuming the volume will end with. Usually those are treated as major events in manga, which is why I think Naoi will give Kurumi the answer to her confession then. Maybe Kokoro will give Naoi a pep talk, or give Kurumi some advice on how to deal with Mona.
Of course, even if Kurumi tries to turn down Mona's request, or tells her she'll think it over, it's possible Mona becomes more proactive and shows up to their school festival, since the festivals are public events, which would likely result in some kind of incident.
New chapter is scheduled for next month. Hopefully Kurumi and Naoi can make it out of this volume okay. Only 3 more chapters left in this volume. The festival arc continues.