Honestly, it'd take me a while to figure out how events flow from the night of revelations to the Grail War, then figure out how much I can put in the prologue, time skip, and satisfactorily show instead of tell what had happened, but wouldn't fit in the prologue. But if I do have the story go down the route of Sakura being Shirou's friend, he would know about the lustfulness that the worms cause, and if I make Shinji's circuits fade quickly, Shirou would have to have weekly sex with both Shinji, and Sakura, (possibly at the same time for maximum awkwardness/ sharing the od drain between the two guys.)
Why yes, I do study at the Lois McMaster Bujold school of plotting.