Believe me I like all HCs of Giyuu being sad or angry at knowing Shinobu's plan but speaking strictly from canon pov, Giyuu would be one of the most understanding ones. Would he agree with her? Maybe not, given he did enquire why she was not at the training or that she was growing paler and from the look of it, he was the only one among Pillars to do so. So, I am not saying he would not try to make her see reason or ask her to go for an alternative given at some point he did care about her well-being. I am saying that he will understand by the end of it, maybe after a long conversation, maybe reluctantly but he will understand why Shinobu is doing what she is. Cruel or not, his own not-that-old inferiority complex might actually make him envy Shinobu's conviction. In fact, I feel like he will act more like Gyoumeiβ unapproving of her plan but amenable. So canon Giyu would understand her motive even if he isn't on board. However, you know who will go absolutely feral though? Sanemi.
Because we know canon Sanemi disagreed with Kanae having her younger sibling in corps. And also what Shinobu is doing remains one of the worst nightmares of Sanemi for Genyaβ sacrificing himself for the corps. Sanemi would go ballistic because Shinobu putting her life up is his worst fear for Genya. Moreover, Shinobu unwittingly be doing something he fears alsoβ leaving behind a family to mourn after her. Shinobu's sacrifice and its aftermath is precisely what Sanemi fears for himself and Genya.
So, he would CONFRONT her. Ask her if she has lost her mind, if she is trying to upend everyone else's life. And given the way Sanemi is, he will never get on board with her plan, not even to the point of understanding.
I think another person who would be cynical about this plan is Obanai, though for very different reasons. Similarly, Tengen would be also harsh about her decision and in fact I think Tengen would also go out of his way to convince Shinobu to not do so. Mitsuri would be more or less helpful. But Sanemi would be the most volatile one, he would not see any conviction or notion for her to go on this path. And I have a headcanon that everytime Shinobu and Sanemi would meet after Shinobu's plan is revealed, they will argue to the point of other corps members needing to intervene!
So, if I have to rank severity of refusal to accept Shinobu's plan, I would say:
Sanemi>>>>>>>Obanai>GiyuuβMitsuriβTengen> Muichiro β Gyoumei
Of course, no one is gonna agree fully but it is more about who will be most confrontational about it, and it is going to be no one else but Sanemi, and surprisingly, Obanai.
