From what I read and have seen on youtube videos, the witness testimony helped the defense more than it helped the prosecution.
That lady who signed a note she could not even read, if she can not read what she signed, what will she do? What other false statements is she willing to make?
The lady trying to diminish zimmermans head injuries told me the prosecution was grasping at straws. It is not how bad the injury is, but how the victim perceives the injury. If zimmerman was being beat down, and "thought" his life was in danger, he could use lethal force to defend himself.