Welcome Sandstorm, great first post.
Your theory is pretty much the way I view what could have happened as well. I would add a couple of other things too. If Lauren were in the shower, would he have been able to hear that in his apartment? I remember long ago when we lived in an apartment you could hear water running in the pipes next door. Or I think he would wait until he thought she was asleep.
Also, regarding the timing. He had obviously known Lauren for a couple of years correct? She was moving to ATL that very week. If he had been fantasizing about her his chances to fulfill them were dwindling. He had the perfect opportunity, they were the only two tennants, there would be no one else to hear a possible disturbance or see him moving things out of the apartment. McD as a law student would have particularly good insight into crime scene evidence.
I wonder what the "personal" items were that he stole from the other apartments? Are we talking panties and bras?
You are the anchor to my soul, you won't let go ...