I feel like the senpai isn't the type to go out with someone to make yuu jealous
Well, she sure knew how to sweet talk Saeki, so I can see her doing exactly that just fine. She's the one who has to ignite the fire somehow after all.
Also, Jade, just because she rejected a boy because she didn't feel anything will not stop others from trying. Have you ever heard of people stop confessing to someone because they had rejected someone else before them? Nope. And in this story, which seems to be quite mature, we could see a confession from someone else to help the characters figure things out but not in the, I'll go out with them first before I decide on my feelings for that other person."
Well, yeah, but I mean that from a story telling perspective it wouldn't make sense to repeat the whole process again, especially when it's been established that you either make her feel special when you confess or there's no going out with her. In any case, a confession can happen, but do note that the guy could be looking either at Yuu or at Nanami in that last panel.
I consider her state of mind has changed since the confession she had in middle-school, where she rejected that boy.
She might even accept that one to assess her own feelings. Yuu strikes me as someone really analytic of herself and her emotions. So she might want to experiment and check if she can develop feelings for that boy, while making Sempai jealous. And Sempai's reaction might move her more than she thinks and realizing that hurting Sempai hurts her too.
The experimentation thing I could see happening, but it I don't think it would help her with her "no giving a crap about confessions" problem.
Her state of mind's development is really interesting anyway, cause sometimes she looks really angry at Sempai's newfound stance of love, yet lately she's been really nice to her, so anything can happen, really.