The shovel being returned after she realized how hard it would be to dig a hole makes sense. Here is what I think happened, this is the same location that Tony says she called and says she ran out of gas. He described it as a back route she takes to cut through the neighborhood. When he got there she was walking to the Anthony's house, he says they went to the house, pried the shed open and went back for the car. She was insistent that she put the gas in herself. Tony says this was the 23rd to his recollection, but her text messages say it is the 20th, and the neighbor saw the green "truck" and a guy there with her on the 20th. If she did truly run out of gas at this spot on the 20th, she couldn't walk away from an abandoned car with a dead body in the trunk, so she walked in to the woods and dumped the bag. She clearly went in just far enough to not be seen from the road, the same distance our peeing utility worker would have gone. If Tony's date is mistaken and it is the 20th, that fits really closely with the forensic reports as well.
I wonder if she borrowed the shovel on the 18th to pry the shed open but couldn't get it done? Tony doesn't say whose idea it was to pry the door open, just that he did with his golfclub if I recall correctly.