Welcome! And that wallpaper is almost as epic as the guitars.
Very sorry for your loss. I'm glad you were able to rescue his collection and all the treasured memories attached to them. Thanks for sharing your story (and these amazing pics!) with us.
I think the sunlight helped, yes. The UV specifically. I scrubbed the interiors lightly with a brush and a borax solution, let them dry in the sun for a few days, then vacuumed the borax dust out. A few visible spots remain but the mold itself hasn't come back yet, a few years later.