I've been trying to retroport Thrallshadowlands as of late too (with no luck). But, in doing so, I've found on Wowtools that he does have 3 LOD .skin files as well, though his m2 doesn't seem to be dependant on them. Maybe try looking on that website's database for the extra skin files and see if you can edit the m2 to refer to them or something?
thrallshadowlands_lod00.skin
thrallshadowlands_lod01.skin
thrallshadowlands_lod02.skin