In the first trial scene when Lightman is on taking his oath, he explains to the judge why it's impossible to tell "the whole truth." It's a mid-range shot, where Lightman's right hand is on the Bible he's being sworn in with. There's then a cut to a long crane shot looking down at Lightman. It's only for a second, before we cut back to the previous shot, hand on Bible. In that interjected crane shot, Lightman is gesticulating with both arms free.
Zoe doesn't need permission from the judge to treat Lightman as a hostile witness. Any witness called by the prosecution is presumed hostile to the defense, and vice versa. But furthermore, all "treat the witness as hostile" actually means is the attorney is permitted to ask leading questions ("isn't it true that...?")
An attorney is not permitted to state in court his/her belief that the client is innocent. That is the question for the jury to decide.