first you have to be pretty turned on to feel pleasure from it, so no imediate finger lick and stick.
Once she is arroused then the clit (which if you know the anatomy swells INSIDE her) and makes things very sensitive. Then you can insert your finger right behind the bone and yes it is the 'rough' feeling tissue. This swells as she is turned on, swells with ejaculant which she may/or may not be able to 'squirt' out.
So you rub that area you should feel it even swelling you want to do the finger like 'come here' NOT just straight in and out. She will probably feel like she is going to pee and that is ok, the g-spot is tissue that is served as a protectant to the urethra for when you are having sex. If it was not there then the penis could cause damage to the urethra.
No not all woman can ejaculate from internal stimulation alone but I think the 30% would increase if she was being played with differently....as for squirting that depends on where the urethra is more times the woman ejaculated 'internally'. (see other post on squirting)
Eiri-never heard of a woman not having a g-spot since it is moreless part of the body...where did you read that? interesting.