I think it was a good question about if they sneaked in through the garage, but I agree with you AzGirl that if LE knew this was the case they would have surely informed us at the meeting or in the reports they gave to the news.
At this point I'm not sure, unless they found an open door or window, if they even know how the perp got in. They did say "no forced entry". So they didn't break a window, or jimmy a door, etc.
My best guess is this was someone she knew quite well and had no fear of in the slightest. It could have been an acquaintance or former school mate or someone she had a few dates with that she actually didn't even know had a "fixation" on her.
It could be this person was a complete sociopath and she didn't have a clue.
Most serial murderers, I have read, were sociopaths who actually charmed their victims into a false sense of security.
But I actually don't think it was any of the above. I think this was someone that Allison knew very well and there was something going on between the two of them. Like one of the elements - the conflict. I certainly don't want to sleuth Allison because she seems like the greatest girl in the world, which makes this even more difficult. But we don't know all her secrets. And for all we know this guy might have been one of them.
I think this is why we can't rule anyone out. Not at this point in the game. We can't just go on leaks that were in the news for one day either. We need to bang our heads against the wall on this one because I think the DNA was destroyed and LE might have hit a brick wall.
I hope I'm wrong.
IMO