Your flannel is beautiful.
I use rice. Your topic reminds me that I need to make some more bags. We have been using heavy mens' cotton socks filled with rice and tie a knot with the top of the sock. It works so well.
We often put the heated socks (2 minutes for us) in our bed before we go to bed on winter nights to warm the bed... oh, so nice to crawl in. Please have your Mom test with different heating times. Often I cut the time down to one minute depending on what part of my body I am laying it next to, especially if it is directly on my skin. Rice is so wonderful and holds the heat for a reasonable amount of time. I hope your Mom's feet are nice and comfortably toasty this winter. Bless you. :-D