1. I believe the unified plumbing code does not allow for PVC under constant pressure above ground.
2. Again, I think the upc indicates a minimum of 18"
3. With PVB, it's not the height above the ground that matters but height (6" I think) above the highest sprinkler. So if you live on level ground, 1 foot should be sufficient. But if you live on a hill, the PVB needs to be at the top of the hill and then 6"-12" higher.