"keep the pressure differencial at a manageable level for the proportional valve. ?"
the output pressure minus by input pressure, is pressure differential.
a typical regular on/off solenoid, the pressure differential is 0-115psi(or higher, working pressure), a proportional valve has less pressure differential(working pressure).
I think the clippard proportional valve has a working pressure 0-25Psi, if I remember it right. so there are two other valves keep the (input-output) pressure at a certain number that not excess 25 psi, and make the flow rate adjustable at the proportional valve.