To a large extent, you can use whatever NodeID you want (0 < NodeID < 31), but choosing one of the standard ones is more sensible than picking a random number and then having to change everything to fit. The major concern is the Node ID identifies the source of the data, so if you have two the same, the data mixes together and can never be separated.
NodeID = 10 has the node name “emontx1”, so it’s a good one to choose. Likewise NodeID = 9 is good for your second.
Have you read this: Learn→Electricity Monitoring→AC Power Theory→Use in North America→Use in North America
That tries to explain how you should set up your hardware.
Our convention is imported power & energy is positive, exported is negative.
They should match exactly - but they won’t, due to component tolerances. Again, like you tweaked "scales = " for the c.t., you can tweak the corresponding number for both to read the same as your meter. But it won’t affect the power calibration! The calculation of power is done in the analogue front end of the emonPi and in the emonTx, before it reaches emonHub. If you’re reading roughly the right number, don’t mess with the USA config setting. That’s just a flag, NOT a value. Logically it should have been USA = true.
You can read the description of how it “works” here (if you want to get totally confused 
): Help EmonPI Questions
Yes, I know the USA voltage is 120 V, 110 V is a common voltage here for controls and portable tools, and the UK voltage is still centred on 240 V.