Do a search for "married segments" or "favored availability" and you will find your answer.
NW will sell the seat from DTW to HEL but not just to AMS. And if you find DTW-AMS-HEL availability then try to add MCO-DTW you might also be unpleasantly surprised. You might need to search for the entire routing or just call.
Agreed that married segments can have different availability from the single segments but it is a good start to find open space separately, especially the TATL segment. But for something so 'complicated' you'll need to call nowadays. That used to be different but let's revisit that. Until Oct. 24 you may also try Continental.com which has a great search engine and try your luck there.