I agree that it's mental. I believe that the mental is a result of the injury still nagging him, but that could be wrong. He could just be in his own head too much.
I also agree that defenses have made adjustments, but a lot of that is our inability to run the ball inside consistently and our reluctance (for some reason) to throw slants and other quick routes over the middle. So, defenses sit on the sideline throws and trust their DB's one-on-one deep because they don't believe Quinn can hit them (which has shown to be correct). All of this is to say that some of Quinn's struggles aren't entirely his own making. If we had CJ Baxter and/or better run blocking, it would help Quinn immensely. But we're having to play the hand we were dealt.
Sidebar: I have no doubt that Arch will be the better quarterback. I expect him to win a Heisman and a championship. But that does not mean that as a second-year player he gives us the best chance to win just because he looked amazing in stretches against overmatched opponents. Who holds the UT record for interceptions in a season? Colt McCoy, in his second year. He obviously became great, and he showed flashes early, but he wasn't ready to lead us anywhere as a second-year player. We absolutely do not know if it would be any different or better with Arch. It's straight up speculation. We do know that we can win with Ewers, because we have. This is actually really simple.