A few thoughts, I was looking at an old article from the Dothan eagle just yesterday and Spivey is quoted saying that heat from the next day prevented officials from declaring a time of death. This link may have been posted but here it is again its from 09. there's also a video with the mothers as well as chief Spivey. how old is the article that gives the 12:30 - 2:00 time frame? if you can remember where you saw it.
http://www.dothaneagle.com/news/article_02c22cbf-dee6-5a21-8207-b17b9cec60c6.html
also regarding why the vehicle would have been brought back with the bodies in it and why herring ave, and what is the forensic evidence that points to the killer being a local. I swear you've been reading my mind again. these questions have been with me for the last couple days.
To me it suggests the the perp didn't want to leave them at the unknown crime scene because its a place he can be tied to. one he's known to frequent, perhaps the property of friends or family maybe even his own. its late its dark he has no tools to hide the bodies low on gas he takes his chances and drives to herring ave. by the looks of it a very isolated spot where once out of the car all he'd have to do is duck into the woods in the event of an approaching vehicle.
I'm unsure what forensic evidence they might have that points to a local. as deca said the perp didn't just happen to roll up on one of the most isolated roads in the area while riding around with bodies in the trunk. It points to the perp being very familiar with the area. They may have some physical evidence but not sure what it could be.