My two cents;
Kubiak leaving had a crippling affect on our rebuild, and Joe's play. Joe doesn't get a pass on his poor play, but he does get a crutch this season coming back from a major injury, and for dealing with the Trestman carry over. He also has to deal terrible line play, and no run game.
That does not excuse his play under pressure, overall. It's been a disaster. But, I imagine he's pretty much done with getting hurried every single play, and having no time in the pocket. This leads to a huge drop in his usually steady confidence, and therefore we see his mechanics go out the door, and his demeanor drop like a rock. The dude is flat out rattled. He's playing scared.
How do we fix this? I think it's pretty clear --- OLine + Run game. We need an OC who will go this route, and Trestman was never it. I think Trestman was a slight desperation move, and Kubiak leaving crippled the organization last season, and again right now. It set our rebuild back by two years. If Joe is throwing 25-35x a game, I think he can be the guy who we know he can be. Until then, I don't know much changes.
This team has a ton of potential over the coming years, but it is going to take a solid scheme for Joe to get his play back on track.