I don't see a way she can do that while being in character. Because putting up a fight means she's either trying to break Rin and Itsumi up to have Itsumi to herself or trying to invite herself into their relationship. Both of which are extremely rude and I wouldn't want to see Kurohime be that type of person.
It is a tricky situation to resolve. The threesome option exists but isn't believable as of this chapter because of Rin's behavior and Itsumi's conflict being about not wanting to hurt Kurohime instead of having romantic feelings for two girls. Mochi Au Lait have their work cut out for them to make this work if they go that route but they haven't let us down yet. At the very least, if Kurohime is rejected then she can find someone better than Itsumi who wouldn't cheat on her. Really Rin deserves better too but she's loved Itsumi for so long and they've been so intimate that a breakup would be brutal on her even if Itsumi is in the wrong.
Now, if Itsumi tries to pursue the threesome route there's a funny way for it to bite her in the ass. It's important for the relationships in a poly to be equal so Rin and Kurohime have to come to love each other as much as they love Itsumi. Now imagine if all the skeletons Itsumi has been hiding in the closet come out after that happens. Rin and Kurohime could then decide they've had enough of Itsumi's shit and kick her out of the poly and continue to date each other. That would be karmic.
No, it’s not a matter of being in character or not. I believe her own pals during the light music club can offer enough comfort and rationality for her to recover if she fails, since they also gave her the courage to confess. It’s just that I don’t believe she would give up to do anything after that rejection comes. Because just like most of us, if she finds out that her hard work to earn her feelings has been for nothing, she will find a way to have it mean something for Itsumi. It will hurt a lot, just like Ruruka had said, and someone will do something about it, instead of just letting her cope with it within the light music club.
If she accepts her fate like that, there’s a good chance her arc will end, because the main drive for her development has been Itsumi fucking around. Would be nice if she can stay inside the picture.
Also, the first time Itsumi and Kurohime confronted each other, Kurohime showed her undergarments by accident, and decided to strike her with a (fake) sword. Her being irrational isn’t completely out of the picture.
And no need to discuss the outcome if Itsumi herself tries to pursue the Poly route. She herself shot it down here during her serious talk with Ruruka, so we can only hear the opinions from Rin and Kurohime (and maybe a few other girls) from now on.
I can still hope, but Itsumi’s mind is made for now, and I can’t deny that until someone else butts in.
And one more thing. The entire line of this manga is about Itsumi‘s Yuri-themed misadventures in school driven by her and will always bite her in the ass. If she actually settles down and stop stealing girls’ hearts, then the manga will have to either update its focus, or be finished. Those are all big changes, so I’m looking forward to seeing how Mochi Au Lait tackles it.