What exact does the statue on first degree arson say?

While motive is not an issue, "intent" is and generally such a charge requires that the actor "intentionally" start the fire. From your description, the would appear to be reasonable doubt in that area.

Lesser degrees of arson often are based on an "intent" of "recklessness".

How good of quality is the video that reportedly shows the smoke as he is leaving?

What was the fuel source for the fire in the sink? Unless is was a plastic fixure, steel sinks don't burn.

This may be a good case where fire modeling can be used to ensure that justice is served.

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