Yes, they were all attacking targets according to marking. They are all level sync'ed though, I wonder if that has to do anything with it. Yes, I used Heavy Swing before Skull Sunder. No offense taken, but I do have a Level 50 Paladin, I'd at least know that much about enmity combos
Yes a few levels makes a big difference early in the game.

It is actually tougher to maintain threat at lower levels. Once you get more tools (as you noted) it becomes a lot easier.