I know that his HS HC and Mullen both tried working on his mechanics, they couldn't make it work which is why he couldn't win the QB1 job for either HC and if not for injuries at Dallas, he probably wouldn't be the QB1 for the Cowboys.
The one thing all coaches work on with every QB and why mechanics are so important, consistency. Inconsistency makes mistakes and mistakes lose games.
Aikman didn't make that comment about accuracy improving because he doesn't like Prescott. It's been proven that mechanics have to be set in early before instincts take over under pressure. The one thing that's been in Prescott's favor is that, when passing, he's always careful with the ball. He's not a Cutler or George using the big arm indiscriminately.
In all fairness to the QB, the drops and fumbles after the catch should be factored in as well. Is he getting the help that Brady, Brees, Mahomes and Rodgers are getting. I've seen Brady struggle with some of his receivers, not named Edelman, this year too.