Also if she was in the game face down for two turns, and I blink her up, does she have summoning sickness?
Yes, because she left play and then returned to play.
Say you can flip her as a 6/6 hit her with Brute Force making her a 9/9, then using Fatal Frenzy to make her an 18/9 and sacrifice her at end of turn
This works. Fatal Frenzy checks the creature's current power, and uses that for its X value. In general, unless a card specifically states otherwise, a creature's "power" refers to the current power of that creature.