ok darn the mesh thing was my only idea....

is every warning about smoothing groups? try this export setting

correct, freshly import the original fbx
along with your new one from blender

put only the og fbx into your scene

in the hierarchy, select the part you are trying to replace
scroll down in the "skinned mesh renderer" field until you see mesh (like in my post above)

in your assets, click the little arrow to see everything inside your edited fbx

then you can just drag the mesh(es) over to the og fbx

PS sorry for slow response i'm at work lol

this happened to me once even when rig was set to humanoid. i never figured it out so i put the new body mesh onto the old fbx and it worked. what happens if you do that?

select body, it's below the blendshapes

confirm in blender that your transforms are all 0,0,0 and scale is 1
use either fbx all or fbx units scale

also, check the console, what is this warning about?