I'm not going to argue against anything you just said, but I don't see the problem. 810,000 is cheap for QB insurance. Hell, we made a 2.5 million option payment on Poole last year when it was unknown if he would make the team.
Yes, in week 7, depending on what BB's seen from Cassell in practice and any mop up duty, we either activate Fiedler or release him. I'm not assuming Brady gets hurt in week 7, and Fiedler has to ride in on his white horse to save the season. He might not even be the backup for the first couple of weeks. But if Brady (God forbid) gets hurt week 11, I think Fiedler would be a good option if Cassel looks shaky. How much time do you think he'll need to build up a rhythym with the offense? Two weeks, tops.
Or Cassell looks good, and there's no room on the roster to carry a fourth quarterback we release him. Honestly, I think BB could pass out the playbook to the league and it probably wouldn't hurt the offense much. Maybe his shoulder suffers a setback and goes on IR.
Like I said, the only way it works is if Fiedler agrees to the arrangement, so I'm already starting with an unlikely pre-requisite.