One cup bleach per gallon of water if you feel you must treat it. Keep the solution away from fasteners if it is framed. A remaining stain on lumber is not an indication of live spores. Not all biological growth produces mycotoxins. You didn't say when the presence of biological growth appeared or describe the environmental conditions. In any case,provide as much control moisture as is practical. How you take care of the lumber from the time it is on site through when it is enclosed is important. Allow for air circulation and don't enclose it in your wall or roof assemblies if moisture content is 18% or higher. Typically, in SoCal we break down stacks right away when using green lumber (19% or more at the mill) or rip through it really fast. Once framed, it doesn't get drywall until it is less than 15%. Read up on this issue at the APA, WWPA, or at Weyerhaeuser, Georgia Pacific if you need technical references. Good luck.

Non- expanding foam is sufficient. There is no temperature concern for the exhaust. The vent outlet at the back of the microwave keys to the wall vent metal so don't over do the foam to the point that the vent metal is deformed. Clean up excess foam when it has dried. BPI certified general contractor.