Given the scenario details you initially provided I can see how there is "beyond reasonable doubt" that the actions were "willful".

As far as the paper/napkin in the sink, is it possible that this item was carried out of the trash can by the plume and dropped into the sink during the event?

Are there pictures of the fire scene?

What does the original investigative report(s) say about the origin and cause?

Does the actor have anything in his/her background that would constitute a motive?

While motive is not an element of the crime, folks don't start fires without a reason (motive).

James G. Munger, PhD, FIFireE, CFPS
www.qdotengineering.com
W3NFA