While I agree with many of your points I think that the fact that Chan will be in year 2 of what, iirc, is a 3-year contract will make it near impossible for him to sit a #3 overall QB for a year. If he does that it would put him in the position of having 8-10 games max in year 3 (When the Bills would be looking to extend him)with a 1st year starting QB to extend his life as an NFL head coach. Remember, this is already his 2nd kick at the can as a top man in the NFL. If he fails here he won't get a third shot.
Also, if the rookie looks good in preseason, it would make it very hard for the staff to stick with their plan, especially if Fitz stumbles coming out of the gate. I'll bet you dollars to doughnuts right now that any QB that's drafted that high by the Bills starts over Fitz from day 1.