I think that Caylee died on the 16th or close to it. I think she may have been buried in the backyard for a time, moved after the 24th. I don't think she bothered to dispose of the carseat during that period because it actually helped keep up the pretense of Caylee being alive. Really, she could or should have taken it out and said "Zanny" had it for whatever bogus trip she was taking Caylee on but I think Casey just makes up lies for the immediate moment without thought to future scenarios.
As for leaving it in the car when she abandoned it, well didn't she leave her purse and computer too? If she wasn't thinking about how that would look, why would she think about how it would look to abandon the carseat? She doesn't think ahead much if at all. It's all for the minute. What I mean is, I don't think she abandoned the carseat with the car because she didn't need it anymore, she just walked away from it like she did everything else.
For all we know she abandoned the car intending to have one last day of fun and kill herself a'la Melinda Duckett but then just forgot about it in her characteristic fashion and partied on, playing it by ear from day to day.