Anton,

Whatever direction your compressor is located, you need to use the NEMA axis system, as shown in the SM-23 document.

Why are you referring to the CG of the compressor ? That has no relevance to a nozzle load evaluation.

Regarding nozzle diameter, it is not clearly defined in the Code and nominal is normally used. However I have seen the inside diameter referenced. Like I've said, it should be clarified with the vendor.