I'll offer up another absurd possibility to explain the hits on/in the car.
Somebody with access to that car bought something secondhand (Garage/yard/estate sale? No wait, found on Craigslist, yea!). That item came from a house in which someone had passed, and the corpse had rested on/against it for some period of time before the body was removed. I'm thinking small furniture or storage, which would have been substantial enough for somebody to _try_ to pull themselves up on if they were incapacitated (reaching for a phone), AND been able to fit into a trunk.
Depending on that object's size, shape and weight, one person may have been able to move it but had difficulty with it. It could have been balanced awkwardly against the person's legs as it was put into the trunk, or left balanced against the rear end of the car for a period of time (payment is exchanged, smalltalk's made, they look at another item, there's enough time for the scent to "stick" to that area, which explains the taillights and the trunk].
The hits in the yard? That item was either for the yard, or placed there for a while before it was washed down.
------
I was sure Caylee was dead as soon as this story broke, and I still lean that way, but stranger things have happened, and truly, fact IS stranger than fiction. The longer we wait for DNA results to be released, the easier it becomes to entertain other possibilities. I'll post one in a minute.