http://deseretnews.com/dn/view/0,1249,595079881,00.html
Thomas said that she last heard from Lori the Saturday before she disappeared, when she received an e-mail with her new address in North Carolina.
"I'm still kind of numb," Thomas said. "It's starting to sink in."
Mark lies were so good nobody doubted for an instant everything he told them. And he had lies to back up lies. Professional liar. He's been doing this for years. I seriously doubt he's going to lie his way out of this.
I'm sure there is an apartment with that address. MH wouldn't have to make up a phony one. You're pretty good at putting pieces together, so how about this? He was supposed to leave first, with Lori following at a later date. We know he wasn't enrolled, so we can assume that his trip back there was for backround information. He scouted likely places that rented reasonably priced apartments, and gave the info to Lori.
I'm contradicting my last theory, but after reading everyone else's replies, I'm going to guess that he was planning on killing her before the supposed move. The pregnancy was just a fluke. Her questions just moved the timetable up a few days. Her disappearance would of course prevent him from moving or starting school. This would also explain why he was able to hide the body so well. He planned it carefully.
Of course he wasn't completely clever, because phone calls are logged and so are credit card purchases. Right down to the last second.
Is this plausible?