all right now I cannot obviously read all the post on this issue but I'm going to try and guess what I see as to this stain and how it relates.
ICA had Caylee in the tire well, placed the cover over the wheel well but it was protruding up. This is when ICA placed garbage bag on top of it for two reasons, one to hide that it was protruding upwards and to mask the smell of what I believe was the beginning stages of decomp. Now either ICA got her father to help her (I doubt) or after some point ICA knew she had to remove Caylee from the trunk. ICA waits until nobody is home, carry's Caylee to the backyard and places her behind the pool in case someone comes home. She then goes to barrow the shovel from her neighbor, goes out to the car and gets all of Caylee's belongings that was near her, places them in bags, gets garbage bags to place Caylee in, then at dark time, she carries the bags to the dump site. This way if anyone saw her they wouldn't think twice of what she was doing. Probably just that she was tossing garbage out.
The stain I believe is the upside of her body that was pressing on the wheel cover, the car was cleaned out using what I have no idea, maybe carpet cleaner type for like dry cleaning, and rag wiped over the trunk area. *shrug* and MOO