I do think that if you're stuck with a backup QB for a seven game stretch, you have to do two things: already have a QB who's capable of having putting up game winning numbers and accepting the fact that he'll probably suck roughly 40%-60% of the time and you'll likely lose those games. I don't think there's an approach that nets you a high likelihood of winning any particular game, but that approach will get you a few wins over a 7 game stretch if the team's any good.
If a guy is a backup, he probably sucks, but there are guys who are too limited to ever put up game winning numbers (the consistent suckers...those guys shouldn't be in the league unless they're young), and there are guys who are boom/bust types that will win you a game here and there. Remember how Quincy would look like a jv QB one week and then randomly put up good numbers? That's how they managed to win some games under his tenure. Cassel, even as recent as 2013, was a guy who could put up good numbers one week, and scare the crap out of all his fans the next week, but that's ok if you're just trying to play .400-.500 ball to stay alive. That guy might not exist anymore, but I think that's the best way to pick up a few wins during a long stretch because a good team should win when the QB is productive and has good stats. That year he managed to win 3 out of his 6 starts and a 4th game where he had 33 attempts for 240 yards, and it was an almost even mix of quality and crappy performances.
If that guy doesn't exist anymore, then there's nothing Dallas can do, but I do think you need a guy who's capable of putting up strong numbers, just incapable of doing it all the time (or else he'd be a starter), and you basically just have to leave him to it.