By the way, there is an extremely interesting article on fungus here. It is mainly directed at archivists, but photographic materials, including glass, are covered. It dismisses thymol, but doesn't mention formaldehyde, and says at one point that UV light is fungistatic rather than fungicidal ( stops it spreading but doesn't kill it), which I didn't know.

The article also possible answers my original question (how do you tell if a fungus is dead?). Apparently the answer is to add lactophenol-cotton blue solution which stains living fungi blue, but I may need to get my hands on a decent microscope.