I don't know anything about the actual process of doing anything legal about it but it does seem like something could be possible. I think PC has made a very minor issue into a bit more by handling it badly. Admittedly, most of Shiina's viewers and donators don't really care about this much, it's a novelty at best. We don't even know if the art style was any good (I think the artist is kinda meh). But I do agree that not fallowing through with a donation goal is bad and something PC should not be okay with. Initially Shiina said on stream that a replacement commission might be made, but in her later discord and twitter posts this doesn't seem to be on the table. Really, I think the best would have been for PC to commission a new work that's specifically what is okay for them and Shiina and not comment on the not-okay animation. But it's too late for that now and I do think the lack of a statement from management and the "no, now shut up" attitude about it is bad. This donathon goal was approved by management. The skeb request was approved by management. Shiina said how embarrassing it was having to show them. Tenma saw the NSFW and seemed to think is was good. So if management is just saying NO, I think they should at least make a statement if they're not going to replace it.
https://archive.ph/qpvQu (Tenma tweet)
There's a few possibilities why they'd cancel it.
1(as stated by Shiina) it's just unflattering, too Gynecologic, it's bad, it's too much and so on. Okay, well it would be in PC interest to make something else that's more tasteful but still NSFW just to satisfy the dono goal.
2 Management is afraid appearance of Shiina in a explicate video violates Canadian Law and so can't be officially released by PC. Understandable but then 1 it shouldn't be show to Tenma and Lia either, it should be wiped off thier systems and again, 2 commission something else and be more specific.
3 PC now decided no official NSFW is allowed for talents. They should really make a statement about this if this is the case.
4 (hopefully not this) Shiina doesn't want any official NSFW. Really if this is the case she or management should explain this. I don't think most of her fanbase cares very much and everyone makes mistakes. It's really understandable if she just doesn't want that image going forward, but it's something that should be addressed.
Basically, regardless of the reason, because it was donathon goal, it's in PCs interest to address it in some way. Shiina saying "no, talk to my manager" should not be the remedy. Extra scarf raffles are nice but the "didn't fulfil donathon" with hang over her and PC and sets precedent that PC may not fulfil it's stuff (which is okay, just not great for them).