I recall during the time when this was all happening, LE said they were catching a vehicle traveling behind TB's truck, on a neighbouring video camera. That is why (imo) they had been seeking a third suspect. I'm not sure on the timelines of when this other truck was seen, but later, once DM had been arrested, IIRC, LE determined the vehicle following TB's was the same vehicle and color as DM's vehicle.
It is possible(?) that DM let MS out of the rear of TB's truck when they met up with DM's vehicle, and MS could have started driving it/following DM/TB at that time. If that is the case, then it is also possible that MS may not have even been present in the vehicle at the time when TB was shot?
Sad to think about, but if that is the scenario that actually happened then that is likely when Tim realized he was in trouble, or would he already have been shot a short distance from home ? DM likely wanted to use the advantage of surprising Tim, and shooting him before he caught on. That is why I think the awkward turn out of the driveway may have happened as DM reached for his handgun, and took it out of the concealment holster we know he owned. Even if Tim were not killed by the first shot, he might have been incapacitated. It sounds like the first shot may have been a head shot that went through his skull and shattered the passenger side window. I hope that he did expire quickly and did not suffer. It would be interesting to know how far a shot would be heard when the caliber of gun used to kill Tim was fired. Like others have said, even if neighbours heard a shot in the countryside, it probably would not have alarmed them. Sharlene and her neighbour certainly would have remembered, but maybe they had gone back inside, and Sharlene turned on the tv.