As you expand your experience, you will realize there is more to flying than what is described in the POH.
Absolutely. But you seem to think that instructors are supposed to teach the students all there is to know about flying, in any kind of condition or airplane, well above and beyond the POH. I simply don't agree with that. And in fact, if you don't fly the airplane in accordance with the approved flight manual, you might even fail your exam.
It is accepted practice to lean during taxi to prevent plug fouling, particularly at high density altitudes in the mountains.
Well, I don't exactly call 2100' MSL an altiport, so I would personally not apply "high density altitude" procedures to a take-off from there. If the engine doesn't run rough with full power, I would take off with full rich mixture. Which is - not coincidentally - what's described in the POH about these situations.
The same applies to taxi. Depending on the length of the taxi, the ambient temperature, the altitude, the RPM required to taxi and a few other factors, I might or I might not lean the mixture. And if I do, how much I lean is done purely by gut feeling - it's usually about 1/4 to 1/5th of the full mixture travel. At the very least what you call "accepted practice" is ambiguous (how much do you lean in the first place?) and not universally applicable. And not prescribed in this particular POH.
Live by the POH; die by the POH.
If there is information/instructions in the POH, or omitted from the POH, that you feel is life threatening, why not file a formal ASR/MOR or something like that, instead of making an anonymous post on a public forum which is neither read by the authorities (at least not officially) nor by the engine or airframe manufacturer?
I've said it earlier, and I'll say it again: Based on the limited information given, I feel that the cause of the fire was overpriming. And that's something that the POH warns against indeed.
When the runup failed due to rough running during the magneto check, the occupants did exactly what they were supposed to do, according to the POH: Abort the flight. But the plug fouling, not leaning the mixture during taxi, and not knowing the procedure for clearing fouled plugs - most of which is not described in the POH - did not cause the fire.
The fact that a few things happened at the same time does not mean that there is a direct causal relationship between them.