First, never use starting fluid. You're just asking for something to break using that. So let me be sure I understand, and for clarification for everyone else....you're spinning the motor, trying to start it, and you're seeing 600PSI on the ICP and 32-34% on the IPR. Is that correct? Or are those the numbers you're seeing at idle? The reason I ask is when cranking with no leaks we will typically see 1200-1500 PSI and perhaps 50% or so on the IPR. When I see your numbers it's usually at idle and with a minor leak.
Anyway, to answer your question, IMO it's best to use the IPR adapter to apply air. Most any air compressor will work, so long as you can get above 100PSI and have a big enough tank....you don't want the compressor running when doing the test because you have to listen for leaks. So once it's on and air is applied, listen for leaks through the oil fill tube and the CCV port on the drivers side. If you hear a leak which is loud on one side and quiet on the other, your leak is on the loud side. If you hear loud leaks on both sides, you probably have leaks on both sides. If you hear soft leaks on both sides then you probably have a leak in the HPOP area. You didn't state what year you have, so it could be your J-tube or STC fitting, or even the HPOP seal itself. If you don't hear any leaks at all I would then open up the oil filter, allow it to drain, and listen. If you hear a gurgle then the HPOP is bad. That is much less likely on an '05 and up motor.
What are you using to monitor? I'd be interested to see if you have sync and especially what your cranking RPMs are. One thing which is often overlooked is the cranking RPM....they won't signal the FICM to fire the injectors if the RPMs are too low. It's possible that by using ether (which ignites very easily and doesn't need much compression to ignite) you're making it spin just fast enough for the PCM to activate the FICM.