A lot of us feel that TB was shot in the field near his house, but there was no evidence of broken glass there. My guess is that a bullet passed through the glass, shattering it, but not breaking it. This would have been obvious to anybody driving near them, so I believe they drove off the beaten path to break out and dispose of that window. They might have spotted the witness as well and just decided to move. Once the window was cleaned up, the doubled back and proceeded north on Trinity.
This would seem to make all the witness and video testimony coherent. Bullman said he saw the two vehicles pull out between 9:15 and 9:30; the video was reportedly from around 9:20.
My only problem with this scenario is a methodological one. That would mean that they went directly from Bosma's house -- say around 9:10, since the last phone call from "Bate" to Bosma was around 9:02 (I am approximating, didn't look it up), and the video near Wilson and Trinity places them there about 12-15 minutes later -- they must have committed the crime with lightning speed and booted it out of there if they went to the hayfield directly.
This is quite possible of course, but it's a tight timeline. It could be that they were surprised or alarmed by something Tim said or did and responded with unplanned speed and violence. Then they realized they had to get out of there. The rest of your scenario makes excellent sense, and if they did as you suggest, they could have pulled over at a number of spots on Book west of Trinity where it's quite dark and the distance between houses/farms is significant. If they turned their lights off and did their work on the window, it wouldn't likely attract attention. Also, if bits of broken glass were found there, it wouldn't be connected to the crime (in all likelihood).
Bullman testified the police came the next day with search dogs and searched the field. He told them that in the past he had seen vehicles go in there to dump garbage, so the presence of small bits of glass, if noticed, might not have raised suspicion. At that point police were looking for a missing person, so they would have been searching for anything belonging to Tim, for refuse with cast-off DNA, for a weapon, anything anomalous. According to Bullman's testimony, they didn't find anything.
Bullman didn't see where exactly in the field the vehicles came from, but one can speculate, with some confidence, that they hid the Yukon over by the haystacks which provided a natural spot for concealing it. If any glass shards came out of the window at the time of the shooting, they might have been covered by hay fragments (which abound near the haystack piles) and not attract police attention, since they knew there were episodes of people dumping "garbage" there.
Unless one or the other accused volunteers some information (or one of their acquaintances reports something either said directly), we may never find out exactly what happened.
But the Crown's opening statement does say they intend to prove that Tim was shot in the truck close to home, and the likely spot for that is the hayfield. If they also saw Bullman walking his dog they would have been doubly motivated to boot it out of there immediately.
