Thanks for repeating my words. I don't mind black powder weapons, but they wouldn't make much sense.
As for Naphtha: Naphtha is the general term used to refer to flammable liquid hydrocarbon mixtures. So you are right, petrol is a good example for naphtha. And what i don't like about naphtha in the game is that it is produced by mixing two herbs together and not by distilling coal tar, which would make a lot more sense and would be entirely possible to implement into the game.
And regarding black powder grenades: Check your skillbook in game and read lvl 100 throwing weapons description. It got changed to
can throw firework charges
so maybe they will add black powder or something similar ;-)