You're completely wrong on the mechanics...Denard didn't regress in his mechanics at all. He just went from an offense that was 1 or 2 reads then run...to an offense that was NFL progressions.
He wasn't asked to have ANY mechanics under Rich Rod and it was fine...he was running a read-option offense that got WRs wide open. How many times was he asked to drop back and go through his progressions, step up in the pocket and throw a lazer to his target like his name was Brady or Manning? Rarely.
Denard had to do that under Borges and struggled. He also got hurt which limited him as a passer.
How has Devin regressed? He's started what...8 games? He was lights out vs. ND this year, his best game of his career. So he's had 2 bad games. Is that mechanics or is that poor decisions on his part? Turnovers on his part?
He hasn't forgotten how to throw the ball. However, he's always had that "superman" in him. Last year vs. Minnesota he makes that play to Dileo and it works. First game of this year he does the same and finds Dileo and it works. Against ND he does that SAME and makes the worst play in the history of organized sport. That's not mechanics.
Borges can do A LOT of things better, and he's not the best QB coach. But to say these QBs have regressed in their mechanics is a poor argument.
Borges is a MASTER play/formation designer. He's probably one of the best 5 OCs in the country in designing a play. He's not a very good play CALLER. None of that has to do with mechanics though.