Dan--
Thanks for reading and you sensible commentary. Of course, what you suggest makes sense--if the diversion was poorly timed and too early---it would alert the Secret Service and others.
I posit the diversionist was told something along the lines of, "Wait until you hear gunshots. Then, do your diversion." After the first gunshot, the Secret Service is alerted anyway (well, they should have been).
So, I think a diversion plan holds water, if the diversionist is told to hold his shot-and-smoke show until after the first "real" shot.
BTW, this diversion action may explain why so many witnesses describe the first shot as not as loud as subsequent shots.