Doesn’t your approach cause more problems than it solves, esp. when you need morph targets anyway? 
It must be a horror to make clothing assets for a character like that. Why aren’t you using DAZ characters.
They have perfectly deforming bodies with ~94 Jcms. You can even use multilayered clothing without need of chopping up the character into several pieces and the workflow is really easy and fast, once you know what to do. The performance is super duper, because the topology (of the standard low poly character without subdivisions) is very efficient.
MakeHuman is a nice choice too (multilayered clothing, performance, can add custom morph targets, fast workflow, support for bhv animations = endless database of 1000s of animations for free available, mesh ready for subdivision)
“Surface, opaque” is the standard setting of any non-transparent material. Normal map blending sounds nice, will definitively try this out, when I find appropriate normal maps for it, but that won’t help with the shading problem here as it is not a problem with the normal map, but some awful bug somewhere at the core of the engine like the other bugs you mentioned.
At least the shadows in my world are the same as before, probably because my open world map is somehow “primitive” with mostly flat landscape with little vegetation, where I had to turn off some shadows, because of performance reasons.