only the male chest, everything else is at 0
The avatar is in play mode with the blendshape link and armature link active
(seem smooth abs is also on by default but thats doing nothing)
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
Not so fast. You know I can't distribute the harness with the avatar model, everything set-up ready to go. So, unfortunately there's some DIY by the end-user.
And about being aggressive; You're frustrated. So am I, since I can't yet find a solution to this rather unique issue.
The harness is made for the 'out of the box' version of the avatar, which seems to be 1st for VRChat, and I got it working on Resonite. You're the first to come with me with this problem. You did mention you had some custom set up for the rest of the stuff you've got for the Viwi. I don't know what those are.
Another thing I came up against with the Viwi armature was that it had a bunch of animation/pose groups once brought to Blender, which only came to light once uploaded to Resonite. So this might be that as well? Perhaps Unity wants to play those poses on the harness? Or maybe I'm spit-balling at this point, but that's just something that came to me. If I recall, I solved the issue by removing the keyframes of each and everyone of those poses in Blender.