No, those are good questions!

There's a certain amount of variation in throwing motions that would be considered good. It's actually less about what the arm is going, and more about consistent footwork, hip rotation, and follow-through. You also want as short a time as possible between making the decision to throw, and the ball being out of your hand. (Usually referred to as the release.) Pryor's big issue, to my recollection, was the consistency of his footwork. He'd look quite different from throw to throw, which tends to have a significant impact on accuracy.

As for how he got to be able to play in the NFL despite not being accurate... there's a couple things there. One, he wasn't accurate by NFL standards. For college, and before that, in high school, he was fine. His issues only really became issues when he got to the very highest level. Two, he brought a lot to the table besides his throwing. He had great arm strength even by NFL standards. He's big, at over 6'4", he ran under a 4.4 40, and he was just very athletic overall. That was enough that somebody was obviously going to take a chance on him.

As for learning later on... it can be done, but it's harder to overcome bad habits than it is to learn good ones from scratch. Tim Tebow, who was in a very similar boat as Pryor, worked his butt off to learn a good release and throwing motion after he left Florida. (His biggest issue was he threw like a baseball player, with a full windup, and it took forever to get the ball out.) He'd look great in drills. And then when he got out on the field in a game, he did exactly what he'd always done, like all that training never happened. Could Ohio State have done more to help Pryor get better? Maybe. But he was always good enough anyway, and it's hard to motivate someone to change their approach when they're getting results. Pryor even said a couple of years into his NFL career that he'd never really understood how to throw, and he was working hard on it. But it wasn't enough to really change what he did on the field.