Your second point is the most important one. Volek and Bennett developed a nice chemistry together running the scout team offense for quite awhile, and it showed when McNair got injured. This is really a tricky situation. There's no question that McNair is the better quarterback, but he simply doesn't look for Bennett as often as Volek does. Perhaps that will change with Mason out of town. Bennett owners better hope so.
I think this is a bit of a misconception. I pulled the following targets from the play by play data at FBG so as to identify which targets were from which QB.2004 targets from McNair:
Mason - 57 - 26.5% of pass attempts
Bennett - 57- 26.5%
Berlin - 17 - 7.9%
Volek's WR targets in 2004:
Mason - 85 - 23.8% of pass attempts
Bennett - 82 - 23.0%
Berlin - 21 - 5.9%
Calico - 4 - 1.1%
So McNair targeted Mason & Bennett equally, while Volek actually threw to Mason just slightly more than Bennett. Also of note, McNair threw a higher percentage of his passes to WRs than Volek.
McNair's WR targets in 2003:
Mason - 107 - 26.8% of pass attempts
McCareins - 60 - 15.0%
Bennett - 46 - 11.5%
Calico - 40 - 10.0%
Volek's WR targets in 2003:
McCareins - 16 - 23.2% of pass attempts
Mason - 14 - 20.3%
Bennett - 5 - 7.2%
Calico - 3 - 4.3%
Berlin - 1 - 1.4%
Hard to judge with Bennett in 2003. He missed a few games and was the #3 WR. Still, McNair targeted him more often than Volek, and McNair again threw a higher percentage of his passes to his WRs than Volek.
The fact is, Volek isn't better for Bennett than McNair. The perception has to do with the fact that Volek threw a lot more passes than McNair and Bennett had 2-3 big games against weak defenses (EDIT: and the fact that Bennett had cracked ribs earlier in the season when McNair was still playing but was healthier with Volek late). I suspect had McNair been the QB in those games, Bennett still would have had big games.
The fact that McNair is at QB will not have a negative effect on Bennett.