OK, I found the reference to when Hinault let Greg go up the road.
It was on stage 14, the stage where Hinault later crashed:
Lemond actually gained 1:51 on Hinault on that stage, and of course we’ll never know if there would have been the crash if he hadn’t gone up the road. About that crash, though: It was inside a km from the finish, so the rule about crashes and time losses was in effect. What I don't understand is that if everyone in that group went down, what time reference is there to give them? It's not like a mass sprint, where the rest of the peloton goes on. Did someone in that group avoid the crash, or if not, did they give everyone the time of the rider who picked himself up off the road and finished the quickest? If the latter, Hinault and the others would have lost some time as a result of the crash.
On stage 17, where Lemond was ordered to wait for Hinault, Greg finished 2:27 behind behind Herrera (who was second on the stage behind Delgado), less than the 2:25 he trailed Hinault by on the GC at the end of that stage. Lemond had actually been ahead of Herrera when he started talking to the team car, and Herrera passed him at that point. So Lemond probably could have finished at least with Herrera and taken yellow at that point.
Still, even if he had, it would have been close. Lemond picked up a few more seconds on Hinault on the remaining stages, including just five seconds in the final ITT. And in some of the earlier stages in that Tour, Lemond picked up some time bonuses, including one when two riders ahead of him were relegated. Not hard to imagine a scenario in which those bonuses could have decided the Tour.
Another "what if" to consider: GL seemed to have a lot of trouble with mechanicals. In 1985, he lost time in the opening Prologue and in the Stage 13 ITT because of bike problems. Absent those, Hinault's final margin would have been much narrower, maybe around a minute. Lemond also lost time in two ITTs in 1986, though one was the result of a crash.
Not to mention if Greg had not switched teams for that Tour, Fignon's injury would have meant he would have been the main man on another team, and would not have had to hold back attacking Hinault or any other contender.