Ahhhh. I've been exactly there with my IIF and suffered the same desperation. It's not glued in, but the fit of the split spanner must be perfect and then it will unscrew nicely. I stopped trying once I started to really mangle those little slots. Then I went to the thrift store and bought a few large-ish screwdrivers and brought them home to my grinder. (I bought multiple big screwdrivers just in case I messed up, and to make sure I had one of an appropriate size) I laid out the spacing of spanner prongs and estimated depth of the cutout between on the screwdriver with a sharpie, then started with a stationary grinding wheel (the corner of the wheel) to remove some central material. Then put the screwdriver in a vise with the blade up and used my Dremel tool with cutoff wheels to finely remove material and make a 2-pronged wrench that fit the retaining ring perfectly. It was a slow iterative process, and I also had to fine tune the thickness and width of the 2 small blades so that everything fit perfect and snug into the retaining ring. Then, the thing unscrewed so very nicely and easily. If I had to make another tool today, I'd skip the stationary grinder but the Dremel process did take a bit longer. Its just that I have better fine control with the Dremel.
Believe me, it unscrews, but it won't work if you try using two small screwdrivers at the same time. I also tried using dividers, and other makeshift tools. That only mangled the slots. And I also wondered if the ring was glued in, or if it was left-hand thread (which it's not). A case of needing the exact proper tool.