What's the easiest way to get to the ICP and IPR valve?
The ICP sensor is very easy to get to on any 6.0 built after 09/29/03, it's located in the passenger side valve cover, toward the front of the engine. If the ICP sensor is biased and reading incorrectly, it can report low ICP regardless of the actual pressure in the system. When the PCM does not see at least 500 PSI of ICP, it will not command the FICM to fire the injectors.
To test for a biased ICP sensor, you can just unplug it and try to start it. This will default the PCM reading to approximately 700 PSI, which will allow it to start and run if sufficient high pressure oil is in fact preset.
As far as accessing the IPR, it can be reached from the front of the engine or from underneath. Check oil Diesel Tech Ron on YouTube, he has a lot of informative videos on 6.0 repair, including accessing the IPR.