Here's a few thoughts for those without any LE background. When you get a call on something like this everybody who is remotely close to the scene runs hot to get there. Adrenalin pumping like mad. Generally speaking the first to respond will be the primary officer. Of course a supervisor will be enroute too, sometimes, but not always, he will take the role of PO when he arrives. Depending on the call.
6 regular patrol officers do not train together like a SWAT team will. I can only imagine the amount of confusion and chaos that might have gone on before the shooting began. One of the biggest clusterf#### I've ever seen happened at a multi agency felony stop. Poor sucker, armed robbery suspect, at gunpoint by 4-5 officers and each of them telling him to do something different. "Hands in the air," Hands behind your head." "On the ground." 'Don't move." "Turn around slowly." "Get on your knees." "Stand up, sit down, fight fight fight"
Each officer at the OP had a different perspective, different experience, different gun handling ability, and different mindset in how they intended to deal with such a situation. I seriously doubt that any one of them could tell you who fired the first shot. But I will bet that since all of them fired in unison, they all saw the same thing. One might have reacted a hair faster than the others.
The notion that every officer has to empty his gun is not completely farfetched but actually not completely true in my experience. I backed up a situation once that ended in an OIS. The guy and his partner emptied their guns into a car and wounded the BG. I didn't have a clear shot at the BG so I didn't shoot. It kind of reminded me of recon by fire but I didn't think I needed to add to the suppressive fire. This particular BG never fired a shot. He did pull a gun from under the seat however. Nobody said a thing about it to me. The partner caught hell from the rest about 15 rounds fired and no hits, though.