I think Casey's attempt at digging a grave in her parent's back yard was unsuccessful. She may have tried, but it was too big of a job so she gave up and went somewhere a little... easier.
Caylee could have died on the 16th, left in the trunk overnight until Casey could think straight of what to do, tried to bury her on the 17th, then take her to these woods/swamp near the home. The smell should be overwhelming within a few days. By that point, the body would have been there long enough for fluids to seep into the carpet... then made the flurry of calls on the 18th to see if anyone was home so she could go there to clean up the trunk. She wouldn't have done this at the b/f or bff's.. they would have asked what she was cleaning possibly.