Here is the real point about Zimmerman testifying.
There will be a story presented that whatever confrontation took place, Martin was defending himself. There's a good deal of evidence to support this, and no evidence that disproves it. Zimmerman's injuries, who was on top, none of that disproves or even minimizes the likelihood that this is the right story.
Without Zimmerman's testimony, there's no good way to get his story before the jury. With his testimony, his story can be fully developed, but his credibility will be crucial.
The jury or judge will most likely pick one or the other of these stories and if they favor Martin's version as presented by the prosecution, Zimmerman's only chance will be how confident they are in that judgement.
That has nothing to do with presumption of innocence or the 5th Amendment, but its possible he won't win if he doesn't testify.