For the first part, the question would be more for Dale's mother. What excuse could he have given to his mother to ask for a ride when he just drove his (working perfectly) car to her house. Remember, in your scenario you say that he works without an accomplice and his parents are not involved.
With regards to the cell phone, he had to have the cell phone when he dropped it off the bridge around 8pm. In your scenario, He would have had to have had it on him after 6:30pm when he last left the condo or he would have had to pick it up at his parents' house.
a) If he had it at his condo and left with it then the pings would have captured his movements all the way to the apartment complex and on the excursion to leave her body somewhere.
b) If he had left it at his parents' house, then that would have left a solid ping trail as well. And what would his parents have thought if they saw the kids playing with it?
Just as a reminder, you asked everyone to poke holes in your theory. The possibilities open up greatly when you add an accomplice. I personally don't see how it would work without one, but you may be able to adjust your theory or explain the two holes that I mentioned. Either way, it would probably help find Michelle.
If he was involved and acted alone, I think that the possibilities are very limited and therefore the places that Michelle could be is very limited as well.
If he was involved and acted with an accomplice, then obviously the accomplice's ability to move is wider but now you have two or more stories that have to match physical evidence such as witness statements.
