The judge asked the state what remedy it wanted. It had grounds for a mistrial, and could have asked for that. That’s how to unring a bell.
On what basis?
A witness is not allowed to give an opinion on another witness, but Zimmerman is not a witness, he’s the defendant.
Wouldn’t that be rewarding the prosecution for their own ineptitude?
If the prosecutor had made a timely objection, and if the witness had answered over that objection, there might conceivably be a basis for a mistrial. Since the prosecutor didn't make a timely objection, however, any mistrial resulting from such statements would have to be seen as the prosecutor's fault. While prosecutors are allowed to retry defendants after mistrials that are caused either by patently-wrongful actions by the defense or by hung juries, any mistrial which is forced by prosecutor's actions becomes an effective acquittal [if it didn't, prosecutors could simply force mistrials any time things seemed to be going badly]. Even if Judge Nelson were willing to declare a mistrial, and declare that it was the defense's fault, such a ruling would be immediately appealed, and the DCA would probably not waste much time in ruling that Nelson's declaration of mistrial conceded the case to the defense.
I wouldn't necessarily say that such an outcome couldn't happen. Judge Nelson probably doesn't really want anything to do with this case, so if she could punt it and put the DCA on the hot seat as the entity that sets Zimmerman free, punting the case might let her escape from it.