Symmetrize a rigged mesh with shape key set to 1?

Hello all! I’m trying to mod a sock clothing model for game. I’ve done the modifications on one side of the model and want to symmetrize it to the other side.

The models came parented to bones with weights all set and done. There is also shape keys which further modify the mesh. For some reason whenever I go into edit mode, one of the shape keys defaults to 1. Here’s some pictures to show what I mean Here’s a imgur album with all the screenshots I want to show (there’s 5 image). :

This is the model seen in object mode:

This is the model seen in edit mode:
(Can’t upload more picture here as I’m a new member, here’s an imgur link)

The edit mode matches the object mode when regfat shape key is set to 1:
Check the imgur album. I can’t link more than twice as new member…

The second shape key regmeet does something different:
Check the imgur album.

My goal is to mirror my changes (left side of the image, the bigger one).

If I only use symmetrize on the mesh in edit mode, the mesh is nicely mirrored. But the process seems to just delete half of the vertexes, and duplicates them over. This causes the vertex groups of the right leg to include both side. Causing right leg bones to control both side of the mesh…

(Extra info, included here to show what I did, possibly irrelevant) I did a jank method where I deleted all the empty vertex groups (the left ones). Duplicated the object. Renamed the vertex groups from R -> L. Removed all the vertices of the right side from the vertex groups. Then on the original object, deleted all the empty vertex groups again. Then removed all the vertices of the left side from the vertex groups. Then did a mesh data transfer (Topology, All Layers, By Name) from the duplicated to the original. Now all the vertex groups includes the proper vertices, weight, and bone control all works properly.

(Back to topic) However the shape key modifier is broken on the jank mirrored side. It seems to be…inverted? It happened the moment I used the symmetry tool, before all the jank method. Here’s what it looks like with regfat shape key set to 1:
Check the imgur album.

So how do I fix this? Can I somehow remove the shape key modifier from the mirrored side, and re-mirror just the shape key modification properly? Or is my jank mirror method wrong from the start and there’s a better way?

Thank you so much for your time!