Q: Who shot first?
A: The Star Destroyer shot first
in ANH, Tantive shot back in self defense,
. Seriously though, Solo shot first, it's been proven fact since 1977.
Q: How long(time) did the rebels have to launch the counter attack on the death star from Yavin4?
A: The Death Star, which was tracking the Falcon, was to be in range in thirty minutes, so I'm guessing the Rebels had the same amount of time to launch their attack. I'll stick with that answer... 30 minutes!