When was he going to do it on the 31st? Family was going to be there and you have already suggested that he didn't do it because of family being there. Not everyone went to Crivitz. The business was open on the 5th, Earl was around, Barb was around, Bobby was around, those are just the family member's that I know of. I think even maybe Blaine might have been around.
And nope, I don't think SA would have any way of knowing if he was the last appointment or not, this is one more reason why he would have no idea that she wouldn't be reported within hours instead of days, and leaving the RAV4 there in the hopes to crush it doesn't make sense, especially if anyone is suggesting he was going to wait until everyone was gone to the cabin, that was 5 days after she was there, and not "everyone" went. Even after he knew that TH was reported missing, he had from the evening of the 3rd until he left on the 5th, maybe he couldn't crush the car, but he could have moved it off the property.