Although I admittedly never have given much thought to the pay as you go phone, (I dislike calling it a burner phone, as again, many, many, law abiding citizens own these devices), I would have to say that I do believe, because of the timing of TB's phone being turned off and it's relation to his home at the time of shut off, that DM and MS did go on the test drives legitimately but that TB was killed after they parted ways, on his way home. I also believe then, that it would only make sense that the phone in question had to have been in hands of at least one of the people at the test drive, otherwise, how would they know where and when to meet the owners of the vehicles? Unless, I guess it is possible, a friend/flunky of DM's told him, 'Hey, I made you an appointment for a test drive for you at such and such a place and time because I knew you were looking for a truck'.
Or alternatively, since I am now thinking about a flunky, a flunky with a cell phone could have arranged the test drives. Brought a similar looking friend or two, drawn on some tattoos to match the ones he has seen on DM ( which could then explain some of the questions some people have had regarding whether or not the tattoo had a box around the word ambition) and then said flunky could have gone on the test drives, said flunky could have killed TB, brought the body to the farm, and desecrated it, and then could have also had access to DM's hanger to retrieve the trailer to stash the truck in, and could have looked up MB's address to bring the trailer to. That would also fit, as far as I can tell. Then I can see a flunky thinking that leaving a body there was a good idea, anything that would point away from the flunky and to DM would seem like good planning and a great way to cover your tracks, wouldn't it?
I actually think that having a flunky going around arranging sales for you, and likely then handling some other aspects of your business which give them access to personal information, financial signing ability, and possibly access to your properties and belongings as well, makes it far more plausible that DM is telling the truth when he claims that he is innocent, in my opinion. That could explain both the incinerator purchase and the body being found on his property and the truck being found at the mother's house, a trusted flunky could have access to all of those things.
Has the phone ever been recovered, do we know? We know it is in a false name, but that could have been known just because it was put under a Bart Simpson-esqe moniker. I don't recall LE saying that it had been recovered, just that it had been turned off. Does anyone know if, when you break a phone beyond repair, does it seem to be turned off? Also, if your battery dies because it is lost somewhere, would it appear to be turned off?