If I wear light armor mixed in with heavy armor, which one's skill would increase? Does it depend on how much you're wearing?

I think both; just half as much for each.

I assume this depends upon where you're struck.

Both. Think of it as a percentage, of what gets hit most. In her early stages, my sneak archer found some Iron gloves of archery, but wore leather armor. I couldn't enchant that well, so I kept the iron for a long time. My heavy armor went up the most when I was blocking. I guess that makes sense. That was the only time my hands were getting hit.

Sometimes when I'm wearing a priest mask my heavy armor will go up. It doesn't level as fast as the light armor I'm using but it does level.

4 pieces of armor, 3 heavy, 1 light. Every XP point gets divided by four. 3/4 of that goes to Heavy armor, 1/4 to Light.