Okay, I've seen screenshots of Duvahkiln wearing hide armor, but he had no shirt on under it, but in the advertisemants it always shows him wearing hide armor with a dark shirt underneath it, so can the hero wear clothes under their armor like in Morrowind?

Not 100% sure but sorry I think not.

I would assume not considering how they've done the armor this time around. There's really only like 4 pieces now. Head, body, boots, and gloves. And I would assume that normal clothing would count as part of the "body" category.

I'm fairly certain I saw someone wearing a light or heavy helmet, and a hood somewhere recently.
Like.. 90%+ certain.
If you can wear a helmet and a hood, i'm hoping you can wear clothes over and/or under armor.
The hood went over the helmet btw.