ok first i dont think there is a female that is hermie resistant, if you stress a female it will herm, mess with the light schedule and you will get a herm. And chemicals arent the only way to make herm, letting the bud over ripen until it produces males sac is the better method.
yes.. there are females that are hermie resistant... they are hard to find due to so many crossings of strains... a hermie trait that is crossed with a true female with dirty the genetic line.. so it is UNCOMMON to find true females.. BUT they DO exist...
secondly, chemically inducing pollen sacs on a true female is the only way to get pollen sacs from it early enough to pollinate that specific female, or another true female, early enough in flowering to get the healthiest seeds you can get, which in turn will grow strong females...
and lastly, the reason that females will produce pollen in over ripened buds is a natural phenomenon.. in nature (which in turn means it will happen under most circumstances) when there is a large group of females in an area with no males to pollinate them, later in their life they may produce pollen sacs to try to produce seeds to ensure their genetic line survives.. its not a guarantee that the female will by letting it over ripe..
and lastly, just because that female produces pollen that late into flowering does no mean it is a hermie.. over ripening of buds is not a stress.. it happens in nature when no one tends to plants... so thats just evolution and survival traits that most, if not all MJ plants, will do in those circumstances...
a hermie will show hermie traits through environmental stresses or on their own, before flowers ripe... that means they show pollen sacs from temp stresses, or lighting stresses, ect. or they just start producing pollen sacs on their own, well before the buds get over ripe...
chemical inducing of a true female is the only way to be 100% sure that the pollen produced contains only 100% pure true female pollen... and pollinating that female or another true female is the only way to be 100% sure that the resulting seeds are pure true female seeds...