Nobody knows what position he was in when the gun went off. All you can do is guess.
I assume you do not do a great deal of shooting. The slide comes back with enough force to shear a finger off. Imagine if that hit you in the nose? Think about the cycling action of a firearm. The slide moves very, very quickly. In order for it to "hit your nose" it would have to have been fired only inches from ones face. It this occurred, 3 things would be true. #1. GZ, without hearing protection, would have perforated eardrums for a discharge that close. That is a medical fact. If you do not believe it,you can verify it yourself. Have someone fire a 9mm next to your head with no hearing protection......GZ would hardly have been able to hear for quite a while afterwards. Officers likely would have noted this. #2. If the slide hit you nose, you would see more damage, plus likely the imprint of the rear of the slide. It comes back very violently. (as I said, enough to take a misplaced finger off) and #3. GZ's face would have had to been level with TM's chest, he would have had to have the firearm against his face, which would mean even inches away, his face would have tested positive for GSR (Gun Shot Residue).
I want to add, your next assertion will be that it was "recoil" and the entire firearm and not the slide hit him in the face. I will save you the trouble. Unless it was a .357 +P or a .44 Mag revolver (or larger), that is very unlikely. Semi-Auto 9mm's have very little upward recoil as the violent action of the slide and cycling it specifically intended to dissipate it, which is why revolver's have so much more felt recoil, even caliber for caliber.
What I am trying to say in a very nice way is that it seems likely you do not deal with firearms, as practical experience with them, would make you realize that your hypothesis cannot be accurate in light of the facts. I do not know where the gun was when it was discharged, but I am certain where it was not, and that was next to GZ's face (breaking his nose and giving him 2 black eyes), and not the person seen by 2 witnesses mounting according to one, and "strattling"(sp) according to another, the person on the ground, pummeling them MMA style ("Ground and Pound" they call it in MMA)