My take on all of this: In the three different versions of Zack's death presented to the player he is killed by the ShinRa Military police. Both Last Order and CC imply that Tseng wishes to reach Zack before the Military Police to spare him from death. More specifically, to deliver Aerith's letters to him. As to the number of police that killed Zack, FF7 and Crisis Core are pretty much the same on that. After Zack defeats the ARMY of Military Police he's facing the three surviving MPs gun him down. The FF7 version where Zack is killed, more than likely, only depicts Zack's last moments, whereas CC focuses on the entire battle.