At worst Oswald fired the first shot when JFK was clear of all parts of the tree except a few outer leaves that really did not hide much.

So you finally admit that you have the assassin shooting through the oak tree!!
Why would he do that when the limo was about to enter a stretch of open road?
Because he knew he may need to make more than one shot before the car had moved out of accurate shooting range and he shot as soon as he felt he had a clear view of the President. At that point there were only a few leaves which did not prevent an accurate shot. I am not saying he shot when he couldn’t see JFK. I am saying that is the worst case shooting before JFK was not completely clear. And I am saying he was clear of the oak tree by z193 when the first shot occurred.
While we are asking such questions, why would the shooter wait over a full second and a half to shoot after JFK was in the clear? And why would the shooter fire a shot after the head shot? Why would the shooter do that after seeing an obvious fatal shot?
And he reacted immediately but gradually, by turning forward moving left,
Immediately but gradually??

Is that difficult to understand? He reacted within a few frames by turning forward with a blank look, then leaned to his left and brought both hands to his upper chest, then he slipped down with a choking expression, then he leaned forward with his head facing downward.
Please explain to everyone what "turning forward moving left" means.
His head turns from facing right to facing forward. Then his body moved leaned to the left exposing JBC to view from the rear as observed by Dave Powers.