In the Defense scenario, OP fired the gunshots shortly after 3:00 a.m.
In this scenario, I agree that it would not be at all likely that Reeva would have still been barely alive as OP carried her down the stairs. In the DT's scenario (gunshots close to 3:00 a.m.) Reeva would have been deceased before OP had attempted to lift her from the toilet room after 'sitting with her for 5 minutes'.
I don't believe that OP sat with Reeva for 5 minutes before he pulled her out of the toilet room (I think that is a flat-out lie on his part in an effort to use up time from when he wants the Court to believe he discovered Reeva's body to when he was witnessed carrying her body down the stairs), nor do I believe that he shot her shortly after 3:00 a.m.
I believe OP shot and killed Reeva around 3:15 - 3:17 a.m (depending on whose clock is used). I've chosen to refer to phone logs, as I believe these are the most accurate times.
According to phone logs & testimony, Dr. Stipp was calling estate security at 3:15:51 a.m. immediately after the 2nd set of 'bangs' that he identified as gunshots.
I believe this is closest to the time when OP shot and killed Reeva.
In this scenario, OP has enough time to put on his prosthetic legs, pry the toilet door panels off, call Stander and then Netcare as he goes downstairs to unlock the front door, return upstairs to carry Reeva downstairs as Stander, Clarice, Baba, and another security guard arrive to witness him on the landing of the stairs. According to phone logs, Stander, Clarice, Baba, and another security guard arrived at OP's house at about 3:22:05 a.m.
Phone log times can be found at Lisa's (juror13) blog:
http://juror13lw.wordpress.com/2014/03/25/oscar-pistorius-trial-day-15-state-rests/
IMO, OP shooting Reeva at about 3:15:51 is supported by Col. van der Nest's testimony regarding blood spatter analysis and arterial spurts on the stairway walls and the sofa in the downstairs lounge.
OP's claim that he sat with Reeva for 5 minutes is very difficult for me to believe. IMO, it's not plausible that in an emergency situation anyone would sit with a gravely wounded loved one for 5 minutes before seeking medical assistance. As I stated above, I think he said this in an effort to whittle away at the DT's timeframe in which they claim the gunshots occurred close to 3:00 a.m.
The DT's claim that OP shot Reeva close to/shortly after 3:00 a.m. - the time of the first set of 'bangs' - is not supported, IMO, by forensic evidence. The blood spatter evidence of arterial spurts on the stairway walls and downstairs lounge sofa do not illustrate a scenario in which Reeva was shot & gravely wounded 15-20 minutes prior to OP carrying her downstairs.
I think it's logical to conclude that OP shot Reeva moments before Dr. Stipp called estate security at 3:15:51.