Merging two theories -- perhaps it was someone who lived in the neighborhood who thought they'd have more time at home, but something happened to interfere with their plans and, ultimately, the disposal of Alanna's body. If someone was interrupted (perhaps a phone call stating a family member would be home earlier than expected, etc.), the primary goal would be to get Alanna out of the house, so the person might've only been able to drive a short distance to dispose of her body. It's not completely unthinkable that the person was holding the tarp and then just dumped her body at the most convenient intersection where there was no traffic.
Also, if her body was dumped because of panic, it seems to reason that the perpetrator needed to be somewhere (either back at home or somewhere for a meeting), and that's why he/she was careless/hurried when dumping Alanna's body.
I strongly believe the perpetrator is in the neighborhood and not a drifter. If it was a drifter/random visitor, she would've likely been abducted and taken somewhere else. The abduction and body dump were within a mile or so of where she lived, and I believe the assault/murder happened in that area as well. The perpetrator needed some amount of privacy to assault/ murder her and to wrap up her body, but it's summer and more people are outside, so it seems like the perpetrator would need some sort of enclosed space (house, barn, some sort of facility). I just don't envision a scenario in which a perpetrator abducts her, takes her to a location farther away (out of the neighborhood), and drives back to her neighborhood with her dead body in his vehicle (in broad daylight, no less). That just doesn't make sense to me, even if the killer is inexperienced.
IMHO, something or someone unexpected interrupted the killer, and that's why Alanna was dumped the way/where she was.
This is where LE approaching the public through the media would be helpful. Who was home during that time? Was there anyone off work that day who took off extra time after the murder and/or who was acting strangely when he/she returned to work?