Actually the 2.6 days does not indicate the time in the trunk, it's derived from an analysis of the carpet sample and indicates the stage of decomp. of the body at the time the stain was made. This indicates that the body was either bagged immediately and a leak occurred at approx. 2.6 days, or that the body was bagged at approx. 2.6 days, so no further stain occurred. The bagged body could have been in the car for several more days. This time estimate has since been revised to include 1. something days.
So a lot depends on when the handling/bagging of the body occurred and what state it was in. It's entirely possible that KC couldn't bring herself to handle the body unless she covered/tried to change some distressing features.
An immature, cowardly person who had a propensity to lie and deceive her way out of all sorts of problems, both big and small, would certainly be capable of feeling both remorse and sorrow and a desire to avoid any possible blame, even if she did not intend to kill or to be negligent or careless. If she was culpable for the accident in any way, or even just felt that she was, I can see KC as someone who would hide the truth and lie.
I don't think Dr G. is saying the tape was applied before decomp even started, I think she is saying nothing more than it was applied before decomp was complete, because the tape was found attached to the hair mass under the skull, which was why it was kept in situ.
Yes, decomp starts immediately after death, but it starts internally. If Dr G. believed the tape was applied before death I think she would have said so. The implications are that the tape was applied when there was flesh in a stable enough state for it to be applied to - nothing more. If KC did the handling of Caylee's body around the same time that the stain was produced then that would have been around 1-3 days after death. I don't think the flesh would have been unstable at that point. I don't think she 'rushed' for the duct tape, it would have been applied some time during that period before she bagged the body, which is most likely 1 - 3 days after death IMO. A heart sticker on the tape could be deemed a sign of remorse/sorrow.