Maybe I'm being simplistic, but let's suppose that all the circumstances are identical but when OP bashed down the door he did find an intruder - a dead one.
Would a court find that his actions were lawful? He presumably would be defending himself by saying that he believed he was preempting a possible/ probable attack on him.
According to the details in the link below, this would not be considered justification for his actions. Preemptive strikes are not permissible - the attack on you either has to be in progress or very imminent.
That someone is simply in your house is not enough. Even an armed person is not necessarily enough.
Could OP demonstrate that all other avenues to save himself and Reeva were not available to him and he HAD to shoot? I really, really doubt it. There was a panic alarm he didn't hit and there was a door he failed to go through. He didn't even attempt a non-murderous way of peacefully getting him and Reeva out of there.
I think his own admissions are enough to see him convicted of culpable homicide.
IMO, obviously.
http://www.issafrica.org/uploads/CQNO8DUPLESSIS.PDF