Well sure - in a documentary such a thing could be acceptable, obviously there are plenty of docs about 9/11 and other topics with horrifying real imagery in them. There can be arguments about how much horror should be shown, of course - just because footage exists, it doesn't necessarily mean it should be used - but that depends a lot on the point of the doc and the filmmaker's discretion etc.
Werner Herzog opted not to play the actual audio of Timothy Treadwell being eaten by a bear in Grizzly Man, and I suspect that was a good call. On the other hand, Spike Lee used graphic morgue photographs in Four Little Girls, and although that was pretty awful to look upon, I think it was "appropriate" for that doc.
I just remembered the term for the difference between being realistic and being real in a fictional work: Aesthetic_distance. To violate the aesthetic distance is to do something that bursts the bubble of fictional reality that a fictional work intends to create. Really, it's just the fancy term for "breaking the 4th wall" but nowadays we tend to use the latter term to mean characters talking directly to the audience, whereas "aesthetic distance" is more general.
David Mamet wrote about this in On Directing, his example was when a movie shows an actor playing the piano, and does the obligatory tilt from his face to his hands to say "look, he's really playing!". Which makes the audience think "Hey, Will Ferrell can actually play the piano!" And you don't want that. You've just pushed them out of the world you're trying to create and back into the real world.
Admittedly, breaking the 4th wall can work in comedies... a comedy can get away with saying hey folks, none of this is real and we all know it! But a movie that intends to be serious violates the aesthetic distance at its peril.
So in the case of ZD30, just from a creative standpoint if not a moral one - beginning the movie with the voices of real people who are really about to die undercuts the movie's intent to make us care when Jessica Chastain gets fake-injured by a fake explosion. Walk it off, Jess - we just heard the real thing. You ain't actually hurt.
So, you can tell a story about 9/11 with actors and dramatic re-creations, or you can tell the story of 9/11 as a documentary with the real footage and the real horror, but mixing the two is a tricky business that risks alienating or confusing the audience. That works both ways too - we've all seen documentaries that suddenly switch from real footage to a sketchy re-enactment and immediately thought "bogus!"
