> 'can you give me a hand' sounds really hot...although for the ending you could just have it that her parts are rotting and no one else he knows will cut themselves up so he decides to get a girlfriend.
> Your idea is very erotic, but story loses its punch if you continue same pattern too much since you relieve tension to fast.
> I think you should reorder it somehow to do most amputations before those body parts are given. probably she could receive more than one request or it can be some other reasons, and in attempt to satisfy everyone she cuts herself into pieces. Finally everyone gets what they asked for just but not like they expected were story climaxes and as epilogue you can write what happens to her body parts and everything else.
> also cutting of head to fuck mouth is somehow illogical I think better reason is to cut head because remaining body with no limb s and pussy is not much useful at that time anyway. It is easier to take just head as fuck toy than everything else.
> If you want to keep element of surprise and make it easier you may entirely ignore any explanations ,who asked what, besides Shinji asking for hand. Rei will just do everything for seemingly no reason leaving reader wonder why is she cutting her womb when Shinji asked for a hand and later everyone just come into infirmary to take what they requested.
> For example Kensuke enters infirmary and notices Rei cutting her breasts, so she asks if she can take pussy (which was cut earlier) and Rei says "yes, it is over there". this is also good opportunity to explain ho those body parts will be used and make another request.
> As you described in your idea, it was unexpected, because we as readers expect that Shinji is asking for help not for a hand literally, and then it comes as surprise that it is literally a hand.
> If Shinji is not surprised this is also fine. It also works as story twist because readers expect that he should be surprised and that prediction fails because he actually wanted a hand and even asks for more.
> I would also add some surprises, like teacher notices Rei ripping out her intestines and screams "Why are you ripping your intestines !!!" ... I told you I need them undamaged and you shredded them into pieces.
> This is also very interesting idea, You should mention it in your story as if Rei refuses to have sex with Shinji but has no objections to cut and give him pieces of herself. She may even complain about masturbation using her body parts.