Some of my customers on part NCC have been audited.
When I advised them I played safe with the authorities.
If the aircraft was permanently in Europe EASA Land, Part NCC applies, full stop.
Then : If the owner flies as Single Pilot --> N Licence is enough ( "safety pilot" is not paid anyway)
IF the owner flies with a "Captain" or a "Crew" whom is(are) paid for the job this pilot(s) has to have both licences.(EASA + FAA)
In part NCC, you have to maintain the list of the pilots that are allowed to fly, as well as their training records.
So far, in both cases, the findings were not on this issue.
For the one interested, one of the finding was that a copy of the manual was not physically at the "home base" of the aircraft, and one training certificate was not updated ( even though the pilot had performed the said training), in the relevant part of the manual.
The other main one was the record keeping for a customer whom did not want an integrated solution ( and stick with paper), some flights were missing as well as W&B records for over flights.
As you can see, these were not great issues and have been corrected easily.