I don't think any outside coach could understand the playbook any worse than the current Cowboys coaches.
Well, I think Dallas' issue probably lies less with understanding their own playbook and more with just being horrific at gameplanning and making in game adjustments. I'm not sure...what happened, but even by the relatively low standards I think most people have for them, the Cowboys have just been utterly undisciplined on both sides of the ball. Three separate penalties when the Bears were stopped on third down that extended drives is bad enough, but then take into account Dak consistently throwing into double/triple (even quadruple at one point) coverage, defenders not keeping contain, receivers not running the right routes, etc. It just all smacks of a team that whose coach isn't doing what's necessary to prepare them. Add that to Brett Maher being incapable of hitting a field goal all of sudden and, yeah, you've got a disaster on your hands.
And, of course, Jerry Jones will keep Garrett around because Garrett is his little yes man. I almost feel bad for Cowboys fans. Almost. I'd be at least curious to see what Kellen Moore could do as a potential replacement.