Here's my theory why OP cannot have chosen to fire a specific shot to the head, and could not be sure where the shots would hit. This is irrespective of the time between shots and the issues regarding single shots/double-tap.
When OP fired through the door he was shooting blind - he would have a vague idea of the direction in which the bullets would travel but little else. At the point of firing he could not guarantee that the person in the toilet was within a specific area. He could just presume that the target was behind the door.
Let's suppose he took a bit of the top half of the door out so he could see where Reeva was inside. This still does not help him much, as we know he didn't put the gun through a slot, aimed and fired. He still shot through the door. He cannot have his head looking through a slot and the gun in the same position, as he would have shot Reeva directly, and there would be no bullet hole in the door.
Anybody who has fired a gun would know that to aim a shot with any accuracy you need to stick your hands in front of you, and look along the top of the gun, which gives you your line of sight for the target. Again, that is only useful if you can see your target. If not, you may as well close your eyes and fire.
If he could see Reeva, he had to be shooting from the hip, like the old gunslinger's did in the western movies (similar to where they walk 10 paces, turn around and fire).
This type of shooting is notably inaccurate and people are never trained to shoot this way. For this reason shooting from the hip has mainly be consigned to movie-land.
He could be lucky and hit the person on the other side of the door, but he certainly couldn't guarantee to hit a specific part of the body.
What makes the theory that OP could have selected a shot to head even more unbelievable is the ballistics expert testimony.
He suggested that Reeva received a shot to the leg and that is what brought her down.
That could well be the correct order of shots, but again I stress that OP could not know where the bullets were landing.
Let's again suppose he took a bit of the top half of the door out so he could see where Reeva was inside. Again, he would need to shoot from the hip.
His first shot could hit Reeva in the leg, but there is no guarantee that he can hit her in the head with any remaining shots.
He would need to watch her fall, and as she's falling aim a shot through the door with the gun by his side, at the exact time that Reeva's head moves towards the floor.
This type of shot would be virtually impossible.
As yet I can't determine the order of the shots, or whether OP knew Reeva was behind the door.
What I am convinced about is that OP did not specifically choose where the bullets went. This was purely a random nature of shooting through the door.
I believe it would be almost impossible to for OP to have selected a particular shot in order to stifle a scream etc.