1 - b (if you add the word 'millibars' on the end!)
2 - I'll leave that to the experts!
3 - N3 is not considered a vortex intersection. ie. if you depart from N3 in a medium, you can go 2 minutes behind a heavy who departed from the full length. (same with N8 and N10 on 09R)
4 - A319 (lower medium) requires 4 miles behind a B757 (upper medium).