Minazoe, from my recollection, the BF angle was a real longshot. There didn't seem to be motive, and I believe he had a very solid alibi. Now, the ex is a different kettle of fish, except that I think we aren't allowed to go into him much here, IIRC. Then, there was someone at work who, apparently, was interested in her, but the interest wasn't reciprocal.
I know that LE is going in the direction of someone in J's circle, but I don't know as much as they do, so I always fall back onto a worker at the building site. It just seems the most logical (again, knowing only what I know, which probably isn't much): That area was just at the beginning of being developed. On one side was a busy-ish, slightly sketchy commercial district. On the other, woods and unoccupied land, with railroad tracks nearby. It was known to be a hangout for transients. I personally know people who've said that wouldn't have lived there, at that time, for all the tea in China. The condos were only partially done, which led to empty or nearly-empty buildings, and at night, when the mall closed, I'd think it would be a scary place. (J even mentioned this to her family.) At the time she lived there, there wasn't a working security system in place.
So, you have a bunch of workers, probably many of them not long-employed and, therefore, not well known to construction/building management, coming and going. Sure, there were likely quite a few regulars as well, but as anyone who lived there at the time can attest to, in situations like this, many of these folks would be hired on the spot as fill-ins. Day laborer types. There's been a question about people living in the apartment across from J but, really, whether anyone was actually living there seems beside the point. There were many opportunities, I'd think, for empty apartments, apartments under construction, etc., to be used here and there as temporary pads.
So, where is J? If I had to bet, I'd say long gone. There must have been literally hundreds of dumpsters around that area at the time, behind businesses, apartment buildings, construction sites, etc.