I do believe that both MS and DM planned to take TB's truck (if DM liked it and gave the signal to MS that it was a go) and I do think they both were fully committed to murdering and incinerating TB. I think the original plan got changed only in that TB was shot inside the truck. Either or both could have shot TB by simply pulling the trigger too soon before TB could comply with their command to leave the truck.
Maybe the reason the incinerator was not taken to the hangar in advance was because they simply didn't have the time, or perhaps DM didn't want to transport it there during daylight hours so made a decision to get it after the murder when it would be dark and when less traffic would be on the road. I don't recall, or know if it is known what they were both up to before TB's test drive that day, but I do recall that DM had to drive to Oakville to pick up MS and the time for the test drive was initially supposed to have happened earlier in the evening but it got delayed by DM.
Those words, "I'm taking the truck" do sound true to me, but they don't sound like something MS would have suddenly announced to DM and TB. After all, it wasn't the point for MS to decide if and when DM would think TB's truck satisfactory - that would have been DM's call, IMO. Those words do sound very much like a signal from DM to MS that it was time to murder TB and take the truck but I think one or both acted rashly and shot TB inside of the truck too soon.
All MOO.