Is the circumstantial evidence that the Nazis started the Reichstag fire strong enough to state it as fact?
No. They know who did it. He confessed, and the police at that time were independent and not beholden to the Nazi party.
Hitler was always an opportunist, so he made use of the opportunity. But the Nazis were the biggest party in the Reichstag, so we’re in a great position even without it.
If it wasn’t the fire, it would have just been something else to “justify” a crackdown. The KPD had been battling the Nazis in the streets for so long, it was just a matter of time before a Communist took a shot at an official in the new government, and convince Hindenburg to okay the totalitarian decrees Hitler desired.