If I move into any of the new houses with Mjoll, does that a$$hat Aran still follow her there? I really don't want to have to murder that guy again, but if he won't leave her alone then I guess I have no choice. It feels awkward to do it though when she talks about him randomly...

Sooo no one knows? Or no one reads the Hearthfire forum lol. I guess this will be one of those instances where I just find out for myself... yay!

He won't if you install the Skyrim unofficial patch mod. I'd also suggest installing the unofficial Hearthfire update as well. They take care of a LOT of the unaddressed bugs/glitches Beth has yet to get around to. Get them off Steam or Nexus

If you're on PC the unfofficial patch is your solution. If not then, I think I've heard of a workaround. You need to Google a bit to make sure, but I thnk you need to move into Aerin's house with Mjoll before moving anywhere else.