Adding my two cents to this thread:
a) I'm a musician and camera flashes distract the heck out of me. I had to stop playing during a REHEARSAL once because the flash was driving me crazy.
b) if it's really dark, as Jefe says, all the flash does is illuminate the head of the person in front of you.
c) I find what works really well is to let the flash go off when it's covered up with something like a Band-Aid (capitalized correctly?!).
d) yes, the performers can see the people in the first few rows, and I know this from tons of personal experience. I'm not sure how far back they can see, but I once got a sign read in a foreign language from the fifth row.