What blendshapes are active in this image?
I have no idea I checked them and there was no other groups with weights that shouldn't be but if im totally honest if this is not made for vr chat you should say that - your comment "Could it be that there's confusion with bones/vertex groups in Unity, where the harness wants to 'follow' another bone of similar name?"
that's your job to fix. not the customer
this will come off as aggressive but iv basically spent my money on an asset that is either not finished or miss-sold
only the male chest, everything else is at 0