I don't think it's an assumption at this point.
The OP was almost suggesting that Garrett should be seen as some kind of sympathetic figure. Sorry, not buying it. He didn't have to take this job if he thought it was a dead end -- after almost a decade in this organization plus his dad's experience, no one knew what Jerry would be like better than he did.
And if it's truly that impossible, quit and go somewhere else. But Garrett doesn't have the character or fortitude of a Mike Munchak. He knows he won't make four or five million at his next job, so he's going to keep treading water as long as he can, while as you say throwing under the bus pretty much everyone else from his assistants to the QB who repeatedly saves his butt
The "Poor Jason" angle also assumes he would ever become a good coach with the right experience. Nah, I don't believe that. Most people with true talent show it pretty early. Think giving Jim Zorn more time would let him blossom into a great coach? No, it only would have meant more losing for the Commanders. At least Snyder was smart enough to cut his losses there.
But our Jason Zorn? Nope, keep him in as long as possible. Continuity, baby.