>>290328921
>Separate materials require separate skinned mesh?
No, but they require separate submeshes/material slots. You can create and assign new material slots in Blender.
>Only the thing the VRC_Avatar component is attached to is uploaded asset-wise?
Yes.
>Reasonable overall combined texture resolution?
2k, generally three separate materials (base body, clothing, transparent textures), sometimes a fourth. Quest users won't see your avatar anyway unless you change the Unity build target to mobile and reupload it, which is a lot of effort.