I probably would have resorted to one of these when the first efforts didn't work.

With a rotary tool of the Dremel type, and a cutoff wheel, you can make a nice cut about 1/16 inch wide in just about anything. A cut down each side of the handle close to but not necessarily all the way to the stem should make the handle loose.

When the handle falls off, if you can't find a replacement, just lock a pair of these on the stem and you're back in business.

While the rotary tool may cost you $20 or $30 (US) and the cutoff blades are about a buck each, once you've used it the first time you'll probably find something else you need it for at least every week or two for the rest of your (or it's) life. (I do recommend the AC powered kind, not the battery type.)

John