>and a dropped cigarette and maybe some sawdust or something somehow ignited a 35cm or whatever wide beam? And the smokers didn’t notice?

You clearly don't know how fire works and just how quickly it spreads. You also vastly underestimate how flammable the roof material was.

>all the circumstantial evidence points toward arson

Pray tell them, instead of slinging vague provocations. Better yet, if you have such clear evidence, I'm sure you're gonna go to the police or the papers with it, right?