An MP5 is a sub-machine gun. The shooter used the Biretta. I would be disappointed in the crew if they made that type of audio mistake. That would be like having someone drive off in a V8 or Diesel and then later saying they were driving a hybrid. It would either be a retcon or deliberately misleading editing.

For the shooter you are looking for someone that:
a) knew Angel was with Hope and they were on guard duty.
b) had access to the armory and knew it would lure Angel away.
c) has attachments to the tower crew or else shooting Hope would have been easy.
d) had access to the gear bag to take the silenced pistol.

Pippen saying "you!" I can't concretely say is telling one way or another. He could have been reacting to someone he recognized and had not yet seen there, like others have said. Or maybe he was reacting to someone he did see there that came into the room pointing a gun at him.

The only noticeable hole I see in my observation is the speed that everything happened. As soon as that alarm went off, Angel was gone. Just a few seconds later the shooter walks in. Unless this person possesses super-human speed, theres no way the same person that triggered the alarm is the one that shot pippin and the radio. There was a very careful plan there that had to involve more than one person. One to cause the distraction and one to go in, take out pippen, and get out before Angel got back.