I never said plug fouling caused the fire.
You said poor mixture control caused the fire. In the title of the thread even.
If they hadn't done a restart after receiving some counseling on the mag drop, they would not have made a bad hot start with the resulting fire.
If they hadn't gotten out of their beds that morning, the fire also would not have happened. That's not the same as saying that their getting out of their beds that morning caused the fire.
I'm here on this forum to learn from the mistakes of others, because I don't have time (or enough lives) to make them all myself. That's why I'm interested in what really caused the fire. Which is still, in my opinion, the sloppy starting technique - overpriming - on the second (hot) start.
What caused the hot start to be needed is largely irrelevant to the cause of the fire. It could also have been a passenger forgetting to turn his cellphone off and putting it in a wing locker, or a forgotten pitot cover.
Having said that, I do agree to Victorians argument that people may start to develop "innovative" starting techniques on engines that are hard to start. And engines with fouled plugs may be harder to start than normal. Still, that's no excuse for overpriming - something that the POH warns against. If anything, a hot engine requires less priming than normal.
The reference to lean of peak was because you apparently strictly adhere to the recommendations of the POH.
I don't, but I do expect instructors to teach their students to fly the aircraft according to the POH in the first place. Only once they've fully mastered that, should I expect them to teach stuff beyond what's in the POH. So I don't automatically condemn an instructor for not leaning during taxi, if that's not something that's written in the POH.