Seriously, you guys have to really think and understand what exactly is going on.
This case is going to be brought before a jury, and in most cases, a juror will favor an eye-witness testimony over someone who did not actually see the event.. Is it possible that Zimmerman was the one screaming for help because a grown man can have a high pitched voice when under distressed. You can call me all the names in the book but those are two undeniable facts that you cannot dismiss.
I don't think that the jury would be swayed like that - the prosecution would bring out an audio tape of both their voices, compare it to the screaming - one and done. The fact that some guy saw them a couple of dozen feet away in the dark and rain for a few seconds and made a judgement call based off of that - that's not really enough to sway an entire jury, compared to the testimony of the mother, the many other witnesses and what will most likely be some vocal specialist called to trial as well.
Seriously, hands down, we know it wasn't zimmerman, thinking so is being intellectually dishonest - if you want to argue that the defence would try and make that claim? Sure! If you want to argue that anyone will fall for it? You're nuts.