Hi Summer
Rigor mortis isn't the exact science that many books and TV shows would have you believe. It usually starts in a couple of hours, but may be up to 6 hours, depending on things like the temperature (eg in cool water vs out in the sun). It usually spreads from the top down - so the muscles of the head and neck are usually the first to stiffen (eg jaw), and eventually the whole body - but that usually takes several hours. It then fades away in the same sequence as it set in - i.e. from the head down. The whole process can last 2-3 days, or longer if the body is kept very cold (eg in the snow).
Obviously, there are many factors involved, including age, sex, temperature as mentioned, body size, how muscular the body is, etc etc. All of these would need to be taken into account when trying to assess an approximate time of death, for example. And it would only be approximate. Core temperature (eg liver probes) would be more accurate, but still has a lot of variables, not least being ambient temperature.
In terms of "flopping" a body into the back seat of a vehicle, it would be perfectly possible for a big strong male to flip a small-medium frame woman's body (eg Allison's) into the back seat in the time scale that we all assume to be the case here. He wouldn't have kept the body around for several hours before disposing of it, I would have thought. It would have been within an hour or so - but that's pure guesswork again.
However, the rear compartment of a 4WD would be even easier.
In terms of covering or hiding the body, I'm not sure about that model of Captiva, but many of the 4WD's of that "soft-roader" category have pull-screens that slide back over the rear compartment and clip into little latches just inside the tailgate, the idea being that nobody can see if you have valuables in the back, eg bags, shopping, etc.