Just some random thoughts...
With respect to intermediate weight jackets, nylon technically didn't replace leather. The B-10 replaced the A-2 and it was made from cotton twill and alpaca. I'm guessing (without being sure) that these materials were at least marginally more fire resistant than nylon.
I have always believed that the A-2 was discontinued..not because it was expensive...but because it really wasn't a very functional flight jacket...even by 1942 standards. One thing's for sure. My father never liked his. He said it was cold and stiff and sweaty and he had to wear a wool sweater under it to stay warm. He much preferred the "green, cloth jacket", as he called it.
As to nylon...what amazes me is that it remained the primary material used in flight gear for so long. For thirty-something years, from the introduction of the B-15 until nomex was introduced in the middle seventies, nylon reigned supreme. Even early on, evidence must have existed that nylon clothing and cockpit fires were a deadly combination.
AF
Attucus, I'd say that cotton and alpaca are very good in a fire, compared to nylon. Alpaca especially doesn't like to burn, but will shrivel up when exposed to flame, like wool. But either way, both are much more resistant to flash burning than nylon.
I agree, that nylon sounds like an extremely bad way to keep your investment in a fighter pilot warm, and static electricity generated must have been a blast when stepping down from the cockpit.