Shouldn't it be "nincsenek itt"?
"A zeneszerzők itt nem vannak, hanem ott" was marked wrong - is it because here "itt" is negated or is my word order always impossible?
It's not acceptable because, as you said, "itt" has to be negated to match the second clause.
Also, we never say "nem van" and "nem vannak". Always use "nincs" and "nincsenek" instead: "A zeneszerzők itt nincsenek". :)
I used nincs and it was marked wrong. Changed it to 'nem' and 'vannak' and it was marked right??