No way I think he should be fired now. He inherited way too many problems for that. My concerns are much as others have stated and the game in Norman changed my perception of the whole staff.
However, even if he resigned right now, I cannot think of one coach who might come that I am certain would be an upgrade. Sark/Day/Smart/Lanning are at their final college stint and hugely well-paid with easy recruiting. Lane has way too much baggage, and I'm not sure he is a better head coach.
My biggest problem is that the team has not improved during the season. These guys would probably lose again to Tennessee and to Oklahoma. (The VU game was very fluky and they have suffered in the injury department, so I'd view that one differently).
I also agree with those who say if adjustments are being made in-game they are far too subtle for me to notice -- and they certainly aren't paying major dividends during the game. I expected the team to come out of the locker room and throttle VU. Sadly, I did not expect that to happen at OU.
Finally, I have 0 inside information, but believe the stories about Milroe basically being guaranteed the QB job till he didn't want it (and, if so, others may have similar assurances for their positions). I cannot fathom any other reason for not at least trying someone else for a couple of series when 4 was struggling. I mean, you do it with a pitcher who suddenly can't find the strike zone, a guard who suddenly can't hit a shot, a goalie who gives up three quick ones in the backof the net... If that's true, CKD is limited in what he can do on the offensive side of the football.
Note: I honestly do not know that anything would have gone differently with a back up quarterback, it's just that I can think of three occasions where many coaches would have at least tried it.