The defense is allowed to appeal on that basis.
True. So if an innocent verdict is overturned, the entire trial was for nothing and Zimmerman goes to prison? They can’t have another trial. Double jeapordy.
You are correct. If the jury declares him innocent, then it's over at the state level. This doesn't stop the feds from prosecuting, though, as they did against the cops in the Rodney King case.
A complicating factor arises of Zimmerman is acquitted of the major charge, but convicted of any lessor charge. You can be sure the judge will impose the maximum possible penalty on ANYTHING he's convicted of.
For example, in the trial of the Branch Davidians in Waco, the jury acquitted most of them, and convicted four on "voluntary manslaughter" (which carries a max of 10 years). The judge added 30 years on weapons charges (later overturned on appeal).
The jury needs to acquit on EVERYTHING for Zimmerman to get out of this alive (you can be sure he will not survive prison for long).