The line going to the actuator on the fuel rail i think is the fuel pressure regulator. I dont think the vacuum lines for the fuel and IWE are on the same lines. What i would do to isolate a leaking line is to start disconnecting some vacuum lines and plugging one end while connecting the vacuum guage to the other end and see if all the individual lines are holding vacuum. What could be happening is that there is a leak in the seizing actuator and letting vacuum out and letting the other side try to engage. Its going to take some testing to see just where your leak is coming from. Remember to check the check valves as well. But since you know one IWE is bad, i would start there.
Also what you are calling a seized IWE could be an IWE that has a leak big enough in it that when you try to apply vacuum with a tester(guage) that the leak wont allow any build up of vacuum and never allowing that wheel to disengage. Which inturn the leak is also letting the other side partially engage while the engine is working harder and dropping in vacuum. Try some of that and see what happens.
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Everything You Wanted To Know About IWE's