The event creatives get access to assets if they are being “asked” to incorporate a particular IP. If the movie is done, that generally means they’ll get to see an early cut. If it’s still in production or post, they’ll review the script, concept art, set photos, and other materials. Sometimes (actually, frequently), they design a house that incorporates elements of the movie that don’t ultimately make it to the final cut. Dracula Untold had Baba Yaga, who was deleted from the film. The Thing 2011 included the cockpit alien, who was edited out. Hollywood’s Truth or Dare included a church sequence that was reduced to a throwaway line in the final film.
As for whether they get a chance to say no… I’m sure they can push back and try to negotiate, but a mandate is a mandate. You have to choose your battles. (This is especially true if the studio’s marketing department is willing to contribute funds to develop and build the promoted property. Getting money from the studio can give creatives the chance to reallocate budgeted funds into experiences they care more about.)