As to the bathtub way of laundering a quilt. Is there any reason to not use the washer to do this? Put it in the washer, fill as you would if using the bathtub but do NOT let it agitate. Use your hands to squish it around and then be able to drain the water and let it spin the rest. I don't have the hand strength to get the water out of one in the bathtub method so am inquiring if there is any reason to not let the washer be the container for doing this. Of course, you would need a top loading machine, not the front ones that are on the market nowadays.
I pretty much ruined a comforter that my mother had hand-carded the wool batt. I didn't know that until I asked her why it was now flat! It might be that you should have this piece dry-cleaned instead of washing it. Do some investigating. Interesting story of how your grandmother cleaned quilts. Glad you inquired just for the heritage!