I've just recently relocated to the Treasure Coast area and having been on the Florida Turnpike a few times in the last few months it's almost difficult to convey to someone who's not been on it, just what a remote, desolate length of road it is from Kissimmee to Fort Pierce (~90 miles, IIRC), especially so at night/overnight. (Though should point out, this is a little south of there.) I will say, there are a couple of curiosities, for lack of a better word, at least to my eye.
First is that it is a turnpike, which, despite the solitude, seems like a risky place to commit such a crime since when you get off the road you'll need to give both ticket & money to an attendent. So unless they or the perpetrator had one of those 'Sunpass' tags that allows one through the toll plaza electronically, you'd think an actual human being had to have seen and interacted with this person (and vehicle) not too long after the murders.
The other is the going south aspect. Which seems to mean, as noted above, that killer and victims were in the car together for a good, long stretch of empty road and it was only near where it really does begin to be somewhat populated again did the person responsible commit this horrific act. Why did they wait until then? It sounds impulsive, rather than planned (speculating very freely, of course).
Your analysis is one of the best I've seen.
I wonder why this family was driving at this hour and where they were coming from. Of course, they could have driven from anywhere, but they just moved in June from Brownsville, TX, and relatives are only mentioned as being in Brownsville, so I'm assuming they weren't visiting relatives and probably not making a longer trip to visit friends. The other possibility is FL attractions.
Did they drive from Orlando? It's a two hour drive from there to Port St. Lucie, and if they were killed around 2:00 a.m., they would have left Orlando around midnight - seems late with kids. From Jacksonville it's a four hour drive, meaning they would have left there around 10:00 p.m. Possible.
I also wonder why the family moved from Brownsville. What was the husband's job? The wife's job? What are neighbors saying?