They are regular VAN nozzles on this item.
No. Rain Bird offers a pressure regulating stem in a 4 inch model.
For the adjustable nozzle flow rate varies based upon what pattern you set it to. For nominal 30 PSI rate is 3.7 GPM at 360 degrees, 1.85 GPM at 180 degrees and 0.92 at 90 degrees. Flow rate is lower if your water pressure is lower.
I had to purchase a female to female PVC connector at HD. Seems to work fine.
This pop-up sprinkler head has a .5" inlet.