I agree with your conclusion but I'm not sure it's this simple. For one, Diarra played four minutes today, turned it over twice, committed two fouls, and put up zeros across the board everywhere else. Does he deserve a longer leash? IMO yes. But you could just as easily run with the narrative that KO is teaching his freshman that nothing comes easy and that you have to pay attention to the little things even if you have talent.
Carlton wasn't exactly tearing it up today with his 2 rebounds in 21 minutes, plus, he's probably the worst defender of all the big men. All things considered, he gets plenty of time.
Williams can sometimes be even more of a black hole on offense because he can't screen like Onuorah can. Then there's the fact that Ollie clearly doesn't seem to like him for reasons that I'm sure he's not just making up. The kid hasn't exactly played a lot of organized basketball in the recent past and at this point there is reason to doubt he will be part of our future.
13 minutes for Onuorah - a guy that just can't play at this level - is too many. No way around it. But at the same time, when Diarra isn't effective and Carlton is tired, we don't really have a ton of good options. Ollie clearly felt this was his best option guarding the pick and roll and he clearly had some set plays with Onuorah prominently involved as a screener that he wanted to use at key moments of the game. Ultimately, this sort of thing can be traced back to Durham leaving last spring. We knew that was going to kill us and in these sort of games it just does.