You said high humidity and moisture acts as an accelerant. Really doesn't make any sense at all.
Spontaneous combustion can occur in large piles of organic matter, due to heat produced by bacterial activity that can't escape the pile. Hay is often suspected to ignite in this fashion.
https://en.wikipedia.org/wiki/Spontaneous_combustionBut it only occurs in situations where there is a large enough pile to prevent the heat from escaping. I doubt that there was that much dust present there.
According to accounts, they were setting up to do the renovation work, not actually doing it.
The fire was first noted shortly after the work day had concluded.
Was electric lighting rigged for the purpose of illuminating the work area?
If so, how? (We use stand alone, diesel generators with light towers in the oil patch for lighting temporary locations with a very low fire hazard from the lighting. Was something similar in use or perhaps some other arrangement, and was it shut down at the end of the work day?).
Just more questions than answers at this point.
One more: In order to prevent violence, would the investigating agencies, on their own or under command from higher ups
not release evidence of arson conducted or suspected to be conducted by Muslims?