I did consider this but don't think it would be loud enough to be heard by the Stipps (considering where the door is located off the bedroom down a short passage, within the body of the house and away from the window). My original theory had the argument on either side of the bedroom door but I wonder now if it took place downstairs and this is why only EvdM heard it (the front of OP's house faces her house). Perhaps OP wasn't shouting at this stage but Reeva was? Perhaps the door was open (but not visibly so) because Reeva wanted to leave? I still think Reeva locked the bedroom door when she entered the bedroom and OP barged it open. I don't think this would have proved difficult.
My comments on your timeline are as follows. Intended in constructive mode!
First bangs
1. As far as I can see - only Stipps hear them - and they say there were 3
2. There is a lot of damage in the bathroom which could account for these
3. There is no evidence of 4 strikes on the door
Conclusion. There was no first set of 4 bangs. This was constructed by Pistorious. This commotion happens early and is heard by Stipps as they back on to the bathroom.
Second bangs
1. This is the shooting, as experienced by all witnesses
2. It occurs sometime prior to Stipps call to security
3. There are no bats sounds after this - see evidence of Mike N
I think there is no mystery at all.
The mystery is created by the defence. But once you realise there is no evidence of 4 bats - the mystery is over.
The Stipps here some fighting in the bathroom - IMO the time they provide is the best evidence of what the time actually was - sometime around 3am.
Then sometime around 3.15 the shooting happens.
It cannot be earlier in my view due to the arterial bleed downstairs.