Bee -

The problem with removing your indestructible polymer is in the "I boiled it ..."

Plain cast iron could quite safely be tossed on the coals of a good charcoal (or even coal) bed and brought to at least a dull red glow, which should effectively pyrolize (burn off) anything with an organic origin. You stopped at around 200F, and needed 200C (550F) or a little more. Cracking a cast iron piece by too rapid quenching (e.g. dumping a red hot piece into a bucket of water?) could be a possibility, but simple still-air cooling has never caused a problem with any that I've cleaned this way.

(Some, but possibly not all, range ovens can reach temperatures high enough to be effective. Most US ranges indicate 500F as a maximum setting, which should be sufficient; but some may not get quite to the maximum that's on the knob.)

At boiling temperature you were probably just increasing the degree of polymerization and making the stuff tougher, regardless of what "additives" you tried.

Having seen examples very similar to the material described, I think it may have been too "gummy" for very effective abrasive or scrapin' 'n scratchin' methods, and I can understand why you elected the "quicker solution" and discarded it. As the gunk is fairly insoluble, the only thing reasonable to try might have been to increase the temp enough (350F - 400F?) to make the gunk hard enough to crack before going to the wire brush, or going to the above indicated 200C or higher to just burn it off.

John