Not much to worry about. The USCG criminal “background check” is a completely different thing than a “National security clearance.”
“Ratings” or “unlicensed” MMC holders do not need to be US citizens, permanent residency is enough.
A significant number of mariners are dual citizens with two passports.
You definitely want to put US Citizen on your MMC application.
I don’t know whether you should also put down your foreign citizenship. That’s question for @jdcavo , or a
USCG license consultant.
Another question is whether you must list all the foreign addresses that you have lived at for over six months. I’m not sure, but I think you probably do. That may also be a question for @jdcavo or a USCG license consultant.
However, foreign citizenship and a foreign passport often prevent people from getting a national security clearance. Some people are told that they can only get a clearance if they renounce their foreign citizenship.
Living in a foreign country, having a significant connection to a foreign country, and even associating with foreigners, can be issues for a national security clearance.
You would be crazy to give up an EU citizenship. In fact, you might want to consider starting your maritime career in the EU. @ombugge and @Dutchie can probably give you some good info about working in the EU.
To work at MSC (Military Sealift Command), you must be a US Citizen. You also would need a national security clearance. So I think you can probably forget about working for MSC.
Working for MSC sucks, so you are not missing out on much there. @DeckApe can tell you about working at MSC.