But the prosecution will argue that he instigated it by following him and even after the initial punch he could have retreated at any time until the last 40 seconds of the "fight" when Martin was on top of him. So he was able to retreat until the last 40 seconds.
Since you know that this argument is coming, then why not avail your client of both from start to finish -- the full statute. He had no duty to retreat at the time the assault began and no ability to retreat as the assault played out.
Use both arguments -- not one or the other. Why distinguish one from the other and then go with the harder standard to meet???
There is zero evidence that Zimmerman could have retreated after the first punch; or that Zimmerman threw the first punch. The prosecution can't substitute "could have" for evidence. It can't even substitute circumstantial evidence for evidence.
Furthermore, it is a black letter error of law to assert that the act of following (even it is intended to close distance) is provocation that justifies the use of force. The prosecution can argue it, and the trial court might even accept it. But such would be clear error.
The fact that O'Mara is now saying that Zimmerman could not retreat is not apt to change, even if the prosecution says that Zimmerman could retreat, and should have retreated.
Your concern is premised on the prosecution being able to somehow prevail by asserting that the law includes a duty to retreat. That would be a risible argument from the prosecution, faced with the plain language of the law.
O'Mara is not prevented from countering a prosecution assertion of a bogus legal theory (e.g., there is a duty to retreat), just because he says, now, that his client was unable to retreat.