To answer both of your questions (sorry I'm a bit late), stat checks aren't randomised. If you failed it once, going back and trying again won't change the outcome. I believe the only charisma check in the game requires you to have more than 1 charisma, so unless you choose the options that raise it specifically you fail the check.
Reflexes used to be the name for instinct, and if I've missed a section where the stat's referred to by that name I'd love to know what it is so I can fix it, haha.