I've just become aware Wendy and Donna's tragedy. I've read most of these excellent posts as well as your links. Knowing so little, I just had a couple of "newcomer ideas":
1. I was in college around that time and a lot of activity revolved around concerts. So I just began to check online to see what groups might have been touring at that time --- all through the area and down into Buffalo, etc. The dates need to be checked out more specifically and correlated with the timeline, but I think bands like the Grateful Dead, Quicksilver Messenger Service, etc. were not far from Toronto in April 1973. Johnny Winter was in Toronto and Detroit right around those days. I wonder if a couple of guys said they had tickets and a place to crash after a concert used that as a lure?
2. If the male outdoor witness didn't see Donna and Wendy leave, I wonder if they went through the kitchen where they knew someone, stopped, hung out and talked with them, and were given a hamburg and fries before they left? Or maybe as someone posted, they took the food out to eat on a loading dock? I don't know --- the witnesses statements differ.
3. Shot in the back by a fence sadly makes me think they were running from someone and were blocked and couldn't run any further. I know it was a school night but maybe they were planning to go see an out-of-town band the next night, skipping work and school the next day and returning over the weekend? Did the perp or perps misunderstand the girls' intention to just go to a concert (with no physical contact), and react violently to what was perceived as withholding or teasing?
4. I saw this happen in college: If Wendy and Donna were sharing a joint in the car (to sample the quality before buying) did they accept some without paying (in cash or whatever way the perps expected) without realizing how these monsters would react?
Lower-level sellers of pot in general seemed pretty mellow in those days, so if it was related to grass, I would guess they were referred to a higher level person(s) or a real sociopath. In either case, I doubt this would be an isolated case.
Please bear with this speculating. Donna and Wendy did nothing different than any of us did in 1973 as teens, but it seems they ran into a monster. Having a gun from Windsor that was handed around, travelled a 4-hour distance, and wound up back in Windsor doesn't sound like something a fellow student would wind up possessing... JMO