The wire from the NO relay contact goes to the white wire of the NMU to tell it you are in the desired RPM band.
The micro-switch says you are at WOT and enables the system.
The fuel pressure switch shuts off the nitrous if you lose fuel pressure.

So, as long as main power switch is on, fuel pressure is high enough, gas pedal is on the floor, and RPM are in the target band, you will spray nitrous. If any of those conditions are not met, nitrous shuts off.

Without a main power switch, you may not be able to cut nitrous if the WOT switch fails closed.
Without a fuel pressure switch, you can continue spraying nitrous when the A/F is lean.
Without a WOT switch, you will spray nitrous whenever RPM are in target range.

OK cool so far I've got it except for the micro switch, the NMU has a built in TPS. So I don't understand the need for the switch.

The WOT switch is to keep the system from activating while you are getting ready to launch or driving down the highway. If the NMU is already looking at TPS, then the micro-switch may not be needed.

Edit: I'm an Electrician, not a "nitrous guy" so I may have misinterpreted the purpose of the switch.

You have it correct Ark. The NMU has the TPS there is no need for the micro-switch.

That's all good my friend, totally appreciate you time and help. You and Madwi have gone above and beyond to help me sort this out. Looks to me like a done deal, we'll see. Should have the unit by weeks end, everything else except the bottle holder will go in tomorrow.