On the “hail mary” by Staubach (who was a great player, btw): the vikings defender had good position to pick it off, but Pearson managed to trip him up, maybe accidentally, and have a clear opportunity to catch the pass.
The DB did have good coverage. What made the play a “Hail Mary” (unlike the many desperation end zone bombs we see today) is that Pearson was the only receiver on that side of the play and, for a moment, Pearson thought he did not have the catch. It wound up lodged between his left arm and his left hip and he was able to secure it before it fell away. The Minnesota fans will forever say that Pearson pushed off but I do not see that on the replay. The defender lost his balance trying to reach for the pass. (and I’m NOT a Cowboy fan so I’m not being a homer).