May a European NCC or NCO opertator use third-country registered aircraft?
Yes this is permitted
Why are non-commercial operations treated in two parts, i.e. Part NCC (Non-commercial with complex) motor-powered aircraft and Part NCO (Non-commercial with other-than-complex) motor-powered aircraft?
The rules concerning non-commercial operations are developed separately for complex motor-powered aircraft (MPA) and other-than-complex MPA because it does not make sense to have the same requirements apply to operations with an Airbus 320 for example and a Cessna 172. This way, the principle of proportionality of rules is preserved.