One note on the hail mary. I'm 95% sure that Mangum was aiming for the receivers in the back of the endzone. Most hail marys throw to the bunch then you have a trailer wr (or even a guy behind the bunch) there for the tip/catch.
Look at Nebraska's design in the NW game, they have Westerkamp behind the bunch, and Moore (???) in front of the bunch. BYU only sent out 3 wrs, so there was no one in behind the play for the tip.
But given Mangum was throwing into a 20 mph wind, the ball only made it to Matthews in front. So having our guys bunched up with the recievers in the back and then having Gerry/Gifford between the bunch and Matthews seems like the correct play to me. The main problem was Williams not staying with Matthews giving him a damn good chance of catching the ball.
So in short, the 20 mph wind plus Williams watching the play cause the problem (not to mention Gangwish diving inside for no reason).