Furthermore, if Trayvon ran away from Zimmerman, why did he come back?
Maybe he didn't "come back." Maybe he just hid, watched Zimmerman pass by and watched him doubling back, and then confronted/attacked/engaged him.
Could it not be argued at that point that it was Martin who was pursuing Zimmerman?
Sure could. However, Zimmerman never said the kid was "pursuing" him, and Zimmerman is alive to speak. We can probably eliminate "pursuit" but not "ambush" or "jump." However, even if Martin did initiate the physical violence, Zimmerman is likely to be convicted of some charge or other. He initiated the chain of events leading to the encounter; his actions may have been construed by Martin as menacing; and he was the adult in the situation and in at least petty authority as watchman, so he'll be held to a higher standard.
And then comes the civil suit.
If Zimmerman is cleared, criminally, and found to have acted in self defense, then Martin's estate is barred from obtaining a civil remedy.