I don't agree with your theory as to the cause of Zimmerthugman's actions. Zimmerthugman had called 911 and cops were on the way. He was told not to follow Trayvon. NW rules state to never approach a suspect. Police tell them at every NW meeting to NEVER approach a suspect. All NW is supposed to do is call 911 and report any suspicious person.What does make sense, is that Zimmerman, being frustrated by the cops lack of ability to catch these guys, followed Martin. As Zimmerman told dispatch they usually get away by running out the back gate, Martin was also running away (from someone following him) in the direction of the back gate (also toward his dad's gf's house).
Knowing the suspicious person would likely be home free if he made it to the back gate, Zimmerman decided to detain him until police arrived.
Why are you following me/do you have a problem? What are you doing around here/no I don't have a problem? However the Q&A went, I believe Zimmerman grabbed Martin, in an attempt to keep him there.
When Zimmerman says he reached for his cell phone to call police,(who were already on their way and were supposed to call him,) I believe he reached for his gun.
With no identification on Zimmerman's part, I believe Martin thought this stranger that just followed and stopped him was going to shoot him no matter what. Since nobody can outrun a bullet, not even this athletic football player, he did the only thing he could. Blind him with a punch to the nose, get on top of him so he can't get to his gun (if he didn't already have it out), and scream for help.
Zimmerthugman should have stayed in his vehicle. He didn't, his Napoleon complex was too great.
The rest of your theory is very likely what happened. Zimmerthugman didn't identify himself to Trayvon.