Actually I've been casting and shooting cast bullets in revolvers for nearly 50 years now.
Many of these bullets cast were from salvaged bullets I've dug out of dirt backstops at numerous ranges around the country. In examing these bullets from probably thousands of guns in many different calibers I don't recall ever seeing one that ever showed any signs of being melted or damaged by the heat of the burning powder. I've also picked up lots of bullets from the range here in Alaska when the snow melts in the spring; most of these bullets look like new other than being fired as the snow stops them without damaging them. Again - no signs at all of any damage from the heat of the powder. I have found numerous bullets however that showed evidence of the powder granules being blasted into the exposed lead base of the bullets but again there was no sign of heat damage.
I suspect that the plastic residue in your bores is from friction and smearing. If you check the edges of the sabots or wads on shotgun wads I think you will agree that there is no signs of melting. Heck - even look at paper patched bullets or the cloth patches from your muzzle loaders- they aren't even scorched!
Cutting my hot gases and/or high velocity particles is a destuction process. I've seen high velocity air with water particles in it cut right through thick forged steel elbows on pipe and high pressure gas wells tht produce sand will cut out piping in a very short period of time. High pressure steam is a bad actor; I've seen it cut through hardened steel in a few hours or less. The throat errosion could be caused by hot powder particles as I mentioned before or perhaps something else or a combination of factors. i really don't know.
I suspect any lead evident around the gap in a relover comes from particles sheared off the edges as it passes through the gap but there is no way to tell where it comes from in my experience.
I don't know why GCs would cause any faster gas cutting of the top strap - I don;t recall hearing that before but I may have forgotten. At first though it sounds similar the Lil'gun errosion talk.