If you can't retreat, it doesn't apply.
"the person reasonably believes that he or she is in imminent danger of death or great bodily harm and that he or she has exhausted every reasonable means to escape such danger other than the use of force which is likely to cause death or great bodily harm to the assailant"
Right, duty to retreat only attaches if it is possible to do so.
And just to keep focus, what prompted me to post at all was this contention of yours.
If Zimmerman assaulted Martin, a felony, he cannot later claim self-defense if he is losing a fight he started himself.There are so many permutations of scenario that it's difficult if not impossible to probe them all "in a vacuum," without a specific scenario to analyze.
I simply wanted to correct the impression that starting a fight results in losing the right to self defense. Starting a fight makes you a criminal, and you can be charged with that. But depending on how the fight evolves, the person who started it may regain the right to use force in self defense. Being trapped and beaten, and submitting to an assailant who shows no sign of letting up, if it creates a fear of serious injury (a jury will sort it out later), justifies resort to deadly force to save your hide. Your assailant presumably could have let up, and chose not to.