Shinobu and the push
(warning: i might just be coping).
In the lastest chapter Shinobu made her first confession, and we are supposed to believe that she's being assertive about her own feelings for Yamabuki. Shinobu is in love with him (they are all) which is why she blushed and looked emotional during that confession, she means it, however I don't think it's about her going after Yamubuki.
First, because she didn't get an arc about gaining some self confidence prior to this confession so unless she confesses but say she won't do anything about it, this confession would feel a bit random. Now let's get back to chapter 64.
Shinobu looked at couples and remembered that girls wanted to date Yambuki, then she started asking herself why would she do if he had a girlfriend ?
In chapter 65, it was made clear that Nene can easily give up on things yet this time, she's serious about her acting career and bowed to her father to prove it. Even Yamabuki bowed and Shinobu looked surprised by this. Both of them are clearly committed to make Nene succeed but why did Shinobu had to see this ?
Well, she ties her self-esteem to her professional accomplissements therefore, she is certain that the more the girls get closer to reach them, the more they get closer to Yamabuki and are perhaps, are more "worthy" of him. Nene has been working with Yamabuki a lot, she's already more successful than Shinobu professionally and she's bold.
In the lastest arcs, Shinobu was the one who got to see Nene being jealous several times, she is the only one who knows that something happened in the hotel room, she was also impressed by Nene staying at Yamabuki's place, she was jealous when she learnt that Yamabuki was pushing Nene to go to the acting school... Basically, from her pov, Nene likes Yamabuki and is bold enough about it but she lacks honesty.
Nene is the only girl Shinobu questioned about her feelings for Yamabuki, because she witnessed scenes that showed that she does love him even if she always denies it. Yet so far, Shinobu had never taken any actions to make Nene realize that, until chapter 65 where she saw what Yamabuki did and realized that Nene needed to admit her feelings him to herself in order to be serious about them.
But if Shinobu likes Yamabuki too, why would she push Nene towards him ? Well, she already described herself as "a supporter who props the girls up", and the last arc with the acting school was pretty hard on her and on her self-esteem. Perhaps, from her pov, Yamabuki dedication to Nene's career showed that he favors Nene, (plus he let her sleep as his place, something did happen in the hotel room, ...). I'm guessing that Shinobu is taking Yamabuki lack of professional "commitment" towards her as a proof that he is simply not interested in her (when he actually said multiple times he liked how reliable she was). Maybe that's why she looked sad in the scene where Yamabuki asked Nene if she liked him, because she thinks he's asking because he's interested in her as well.
Unlike the readers, Shinobu doesn't know that Yamabuki is as committed to Nene as he's to the others. Shinobu poor self-esteem, espacially now, and anxiety just don't let her see the whole picture. She doesn't even let Yamabuki calling her by her first name because she doesn't think she deserves it (since she's the furthest from her dreams). So if she thinks she has no shots with Yamabuki, that both Nene and him are into each other at least to some degree since he's more dedicated to Nene's, going as far as bowing to her father and she knows that ultimately Yamabuki will date someone then ...
"Gravity assistance can be used to accelerate a spacecraft, that is, to increase or decrease its speed or redirect its path" .
By "confessing", and probably saying she lies later, she's giving Nene the push she just had, when she asked herself what would she do if Yamabuki had a gf. Putting Nene into a corner just like Yamabuki did with her when he made her admit her shortcomings.
So, you could interpret it as "increasing" Nene speed or as a redirection : making Nene stop denying her feelings.
I was wondering why there was this pattern in the lastest chapters, of only Shinobu seeing Nene/Yamabuki scenes, I guess it could be for this moment.
Also, it's pretty convenient that Shinobu made her confession a few chapters after everyone praised her acting skills.
TL;DR Shinobu confessed hoping that it would make Nene realize her own feelings because she thinks Yamabuki also likes her but she will (probably?) pretend it was a lie.**
End of cope//.