Rush benefited heavily from a great defense. Maybe Grier could’ve too. We will never know.
Probably not, just like we'll never know if other backups would have won even if they benefitted from a great defense. Rush had to make some throws, had to not make some mistakes, had to get lucky, etc. The team was 4-1, and he was part of it. That counts for something.
I tend to think quarterbacks get too much credit or too much blame for wins and losses, but there is a degree of validity when you are the player who touches the ball more than any other player but the center. Bad backup quarterbacks lose games that the team should have won. Rush was a good game manager when that is what the team needed him to be. I think it's wrong to take away from that because we've seen some clear examples of QBs who weren't.