None of the movie titles have used the possessive prefix (の) when they're referring to the feature Pokemon directly if it's placed after the major title of the movie. Giratina and the Icy Sky's Bouquet Shaymin, for a prime example, is 氷空の花束シェイミ. Note that the の is used to join Icy Sky and Bouquet. Deoxys movie was 裂空の訪問者デオキシス (Sky-Splitting Visitor Deoxys, same join here), and so on. To do so now would break the flow, and would be incorrect for the gravity of the title.
So, unless more information pops up to dethrone Zekrom the title of being (the) Dark Hero whom they've just placed at the base of their titles, that's how things are. There are no assumptions here, we're going directly off the news right there in front of our eyes. Can't do much else asides from speculate at that point. You've also contradicted your own argument for the translation (for which, by the way, was translated by someone whom lives there and uses the language essentially natively): "What is the relationship of Dark Hero and Zekrom?" is a re-wording of "What's Zekrom's relationship to being the Dark Hero?"
Both questions ask about Dark Hero and Zekrom, and what relations are there. Yes there is more to learn, but we don't have that information yet. The movie title will be the biggest contradiction of itself if they really play their cards right and he turns out to not be Dark Hero, but until there is concrete proof from the big wigs...