How can I fix the weight painting on this Unity mesh?

Question

I have a character from Unity:

When I export her from AssetStudio, she comes through badly distorted like this, especially in the arms. I suspect the weight painting is messed up, but I'm at a loss on how to fix it. This is the HD version of this character. The LD version doesn't have this problem, so I was hoping to transfer the weights from the LD version to the HD version, but the best I've gotten it to work is that some body parts get corrected while others get worse.

I've also tried clearing the mesh's parent and the re-parenting it to the armature using Automatic Weights; this made it a bit better, but there was still a lot of distortion. I could use some solid suggestions. How can I get this character's weight painting to work correctly?

Here's my .blend file, with both HD and LD versions: Blend File

Answer 1

Transferring weights by data transfer modifier gets rid of most problems:

Where problems remain are where surfaces on the source (LD) mesh are very close to each other: hair, robe, chest are all basically on top of each other, so getting weights from the nearest won't necessarily get weights from the right bit:

The chest is getting weight from the hair mesh. But we can prevent it from doing that by separating out LD's hair to a new object:

Then we can transfer weights from the body, to the body, and from the hair, to the hair, without the surfaces of the two competing with each other:

And, we get immediate improvement.

The more that you part it out, the more specific you can be about which part of the LD you want to copy from. You may wish to part out the HD as well so that you can transfer weights from different parts of the LD to previously undivided parts of the HD. Likely, the next bits you'd want to part out would be the various shawls.