It sounds like you might want to drop $20 on a basic repair manual - available at most auto parts stores. (It will probably be a Haynes. You can get better ones online, but Haynes is ok for the basics).
To test fuel pressure you need a fuel pressure gauge. It screws onto a test port on the fuel rail. For the pressure specs, you get those from the manual, or google is your friend. Make sure you don’t have an external, in-line fuel filter before you ignore something simple.
The IAC generally bolts onto the throttle body. When your foot is off the “gas” the throttle plate is closed. The IAC spans ports before and after the throttle plate and puts the right amount of air through for idling. Those ports can get gunky and the valve itself gets gunky. When you go from park to gear (D or R), load in the engine is increased and the IAC has to adjust a little. If it isn’t then the engine is getting starved for air. You would have to remove it from the throttle body to clean it. You will probably need a new gasket for it. If you can’t find one you can cut one out of something like Karropak gasket paper.
Temporarily taking the vacuum line off of your EGR valve to find out if the symptoms go away will not harm anything. If you permanently disable it you can get pinging which can do engine harm over time. But simply pulling the vacuum line from the valve to find out if the symptoms go away is a 5 minute operation. Plugging the vacuum line - temporarily - is just to make sure that the vacuum the solenoid lets though through can’t act like a vacuum leak and mess up the “test.” Plugging the port of the valve - temporarily - is to make sure that if the valve is sticking open some, it can’t mess up the “test” in the same way. You could actually drive around that way for a while without hurting anything. But - yes - it is a terrible idea as a long term thing.