I'm not suggesting that Constant's theme was bad for the TZ - as a matter of fact it did have that ominous ticking motif that very aptly went with that equally ominous counterclockwise ticking clock that featured in the opening montage, as well as being a subliminal countdown for whatever calamity was to plague the protagonist. I didn't know it was the result of collation though. I have a fantastic (no pun), huge book on horror/fantasy film/TV music called Musique Fantastique (Larson). Maybe the story of the TZ theme is detailed in there, I can't remember.
By the way, all film composers borrow and/or steal of course, but Morricone or Barry for example do that a lot less than say Steiner, Tiomkin, Waxman, Webb, and all that old guard who in all their musical greatness they very much were a poor man's Strauss and Rachmaninov (Wagner really where it all began). Some of them, like Rozsa for instance, do venture a little into "modern" orchestrations and expressionism on occasion, but they are very much rooted on late-romanticism whether the film is a thriller or romance (Herrmann being a great exception, his own voice being too strong and one whose influences were more aesthetically modern). This is not a criticism, just a fact; they wrote great music. Later composers are more varied and eclectic in their musical expressions and so their own disparate styles come through easier (say Mancini, Jarre, Goldsmith, Morricone, Barry, Williams, Delerue, etc.).
Finally yes, all these people were very versed in art music. It would be very hard to compose Casablanca, North by Northwest, Cinema Paradiso, Papillon, Goldfinger, and Star Wars on 'dubstep' and 'gangrene style' principles with arpeggiators and Cine-Ork.
Be that as it may, I have to repeat -reluctantly- that the most original of all must be Hans (again, no pun); Koyaanisqatsi being the closest archetype I can think of (no comparison obviously...)