I guess we could start out by discussing what they need to show for various charges. I think without FBI corroboration of some sort of racial animus or proving that it was TM pleading for his life on the 911 call, murder 2 is pretty much out the window. I think GZ's conversation with the dispatcher doesn't sound like someone about to go on a murderous rampage or even to take justice into their own hands and the I don't see any evidence for them proving beyond a reasonable doubt that it was TM screaming for help.
So to get a charge of manslaughter, what does the state have to show? Obviously they would have to get over any statute that provides immunity to GZ. To do that, they would have to show that GZ was the initial aggressor, that TM and GZ were trading blows in a mutual/fair fight before GZ shot him and or that TM stopped his attack and was leaving the conflict before GZ shot him. I think the injuries (GZ's extensive ones and TM's lack of) remove #2 and witness testimony and forensics on the distance of the shot removes #3. That leaves showing GZ the aggressor. They will start with GZ getting out of his car and uttering expletives about Martin. This does set a table for GZ as the aggressor, but I dont think it gets them there as the defense will show that GZ didn't get out of his car while Martin was near him, but only after he left (confrontation avoidance) and that GZ was calmly talking to the dispatcher after TM was out of sight. They will probably try to introduce GZ's past "violent" behavior with the restraining order by the ex and incident with police in '05. I wouldn't go as far as to say they are irrelevant, but I doubt they are allowed into evidence.
That pretty much leaves Dee Dee's testimony who was an ear witness to the initial confrontation. I think if you take everything she says at 100% face value and true, I think its close on whether this shows GZ as the aggressor. She doesn't detail GZ running after TM or cornering him or coming at him with gun drawn. She has GZ following TM for a short period where TM refuses to run and has TM verbally confronting GZ first. She does report bumping sounds with she attributes to GZ hitting TM, but I dont' see how that passes muster and make it into evidence due to speculation ("I heard bumping sounds" is all that will make it). That leaves the "little bit of get off" that she thinks she makes out as TM's voice which could show GZ attacking TM.
If they can't get over immunity, then they have to prove that GZ didn't fear great bodily harm or for his life. The police report says GZ's injuries are marginally consistent with someone fearing for their lives, so I think that coupled with the photo images and screams for help captured on the 911 call make that a tough task.
I'm not sure what else the prosecution has. If GZ takes the stand, they surely will try to attack his credibility with inconsistencies in his statements. This might work against GZ in the immunity hearing where he has the burden of proof, but in the criminal trial, there isn't any evidence in his testimony that shows him starting anything or that would make one consider that he wasn't in great fear for his life. I'd bet on GZ testifying at an immunity hearing, I'd highly doubt he would at trial. I'd give the prosecution a small chance at winning an immunity hearing (maybe 20%), but I don't see any way with the evidence in the record to date (which includes very little from the defense) that they get a conviction on any charge. Once the emotion of a dead teen is put aside, there just isn't much evidence against him and quite a bit for him.