What if Trayvon DID make it back to his back door and GZ saw him getting ready to go in the door. GZ with the frame of mind he had about young black men in hoodies, would have very likely assumed that Trayvon was breaking into the residence. After all, Trayvon would have no evidence that he was legally staying at the townhouse as a guest and the resident was out for the evening. Maybe GZ attempted to do a citizen's arrest at gun point and started walking Trayvon back to the street near where GZ expected the police to show up, near the clubhouse.
Trayvon could have argued with him and resisted going along. Maybe after going part way down the path, Trayvon realized that this was not LE, just some cop wanna be and simply stopped cooperating. Maybe he pushed GZ down or even gave him a punch in the nose, but there was no evidence on Trayvon's body (according to the mortician) or GZ's body and clothing (from the SPD surveillance video) to indicate a serious life and death struggle.
I could see GZ with his temper (as described by various of his past friends) losing it, pointing his gun at Trayvon, and threatening to kill himn. Then Trayvon would be in serious fear and danger for his life. From ALL descriptions of Trayvon's family, friends, and teachers, he was not an aggressive young man. That would explain the pleading for his life, wailing for 44 seconds.
From the sounds of the wails on the 911 recording, if that is what happened I don't believe Trayvon attacked GZ. Those wails did not come from someone on the attack. They came for someone who was pleading for his life. And they went on for FORTY FOUR seconds, far too long for the shooting to be a spur of the moment event.
If this was what happened, GZ DID commit cold blooded murder. No prosecutor would ever be able to prove it, though.
All of this of course is pure speculation and my theorizing, building on what you have put together.