You said you were seeing movement at the ICV. You do not want this. You will have the PRV connected upstream of any of the plastic pipe. Then the plastic won't be getting hammered so much.
Re-reading this thread, I realized that I did not properly describe how my system is set up.
Basically...the ICV valve comes before the manifold and the rest of the valves (Hunter PGV Jar-top), so the only plastic parts under constant pressure come before the ICV valve, which is simply a couple feet of PVC pipe and a few fittings. There is also one 90 degree manifold elbow and one schedule 80 PVC nipple just before the ICV valve.
Should 150 pounds of pressure slamming into a pressure regulator be OK? Isn't that what it's for?
I guess getting a threaded regulator in after the backflow device would be manageable, I am just trying to decide how necessary it is (the system cost me much more than I have expected already).
Thanks for all the advice.