Hi
Hidden faces or not: it depends on the way the character is exported. With mhx2 "hidden faces" must be switched off. When imported you must eventually overwrite exported data and use "modifier".
What did you use? There are some ways to export a character. MPFB, mhx2, collada ,,,
For skin under clothes some user or "custom" targets are not changing the helper mesh, so clothes may bleed through. Only a few users know that the helper also must be changed. Typically clothes (except from e.g. underwear) are bound to helper meshes. Most "official" targets are more correct but of course will never be perfect.
For the helper:
http://www.makehumancommunity.org/wiki/Documentation:BasemeshWhy it sometimes does not work:
http://www.makehumancommunity.org/wiki/Documentation:MakeClothes_Delete-Groups
@woodcat: same over here ... Germans in Spain (Italy, Turkey ...) in vacation, they always think everyone speaks German as well. But there is help on the way, the lowest common denominator is that the other nations learnt what Bratwurst means and so at least my fellowcountrymen will not starve