I was using unreal back when it was version 2.0. Sockets existed in the engine back then. And the base skeleton didn't look like this. It had no twist bones - for example.
Not particularly, I'm not sure I understand what it is. With Substance painter I regularly release a normal map, basecolor, OcclusionRoughnessMetallic. But I don't have this problem when I import them on max or unreal for example.