As to the tracer rounds, I had thought the first shots were a ten-second rapid-fire burst.
But someone posted a video on youtube from a smartphone at the concert and you can hear a few shots earlier than that, during the second song.
Maybe he fired just a few tracers at the very beginning to get a general sense of angle of fire, and then switched over to regular ammo.
It is said some shooters load one tracer to every few regular so they can get a better aim.