Vinegar is a great, all-natural way to kill mold/mildew. On top of that, it also is wonderful for preventing some dyes from bleeding. I would not dry something that I added vinegar to, but I have washed shower curtains, indigo-dyed blue jeans & cotton bags with 1/4 C vinegar per large load of cold water wash. I would wash it (or soak & rinse) with just vinegar & water the first time & then follow up a second time in the washer with a gentle detergent. I've really seen vinegar work miracles on stuff with mildew.
And I can totally relate to your story about something growing mold very quickly. My storage locker had a pipe burst out in the hallway in the middle of winter & it flooded under the door into my unit & promptly froze everything to the floor. I had to wait 10 days for stuff to thaw enough that I could remove the items that were 5" from the floor or lower. By then, the wood was all rotted with black mold, my leather purses had green mold on them & my cotton clothes & bags had black mold/mildew on them. But other than one purse that my mom decided to "help" treat before I got to it, everything came out with that 1/4 Cup of white vinegar.
I "washed" with the vinegar water, then washed with Tide Free & then air dried and left the items to air out for a couple weeks to remove the remaining vinegar scent. Everything came out perfect with no stain & eventually, no remaining vinegar scent. Might be worth a try. So sorry you are having to deal with this though with your beautiful quilting fabrics.