What's wrong with "Kati harcol a rendőrök és a riporterek között"?
There is nothing wrong with it.
"Kati harcol a rendőrök és riporterek köszt" was marked incorrect. But is it really? Seems to me it could be a good alternate translation.
Yes, it is good.
Köszönöm. Thank you. Good to know. And good also that Kati is standing up for justice!
You are welcome. But, oh, I just noticed, you wrote "köszt". That is incorrect, it should be either "között" or "közt". Even if it may sound like "köszt", it is actually "közt".
Why I can't apply the accusative form on the words rendorok and riporterek
Because that would be grammatically incorrect. Just like adding the -ben or -on or -től or any other case form would be incorrect here.
között doesn't go with the accusative form of words, to the best of my knowledge.