The Motion for immunity will be decided after a hearing. The judge must take and weigh evidence. If the judge finds it more likely than not that Zimmerman was reasonably in fear of imminent serious personal injury (and in FL, a person throwing beer bottles at you is enough to create that fear), then he must grant immunity from further prosecution. The use of deadly force was justified.
On a Motion to reduce the charge from murder to manslaughter (assume for talking purposes there is NO immunity from the judge), there would probably not be a separate hearing, and might be no need for an heading at all, not even the immunity hearing. The judge needs to find probable cause, some scintilla of evidence that Zimmerman operated with a depraved mind, or else the basis for a murder charge is lacking. The state has been asked to produce a Statement of Particulars, which is supposed to tie allegation of fact to elements of the crime.
A Motion to Dismiss based on either ground can be made at any time, even during the conduct of the trial. I don't recall a case that was charged to the jury, where defendant subsequently appealed and won on the basis of being entitled to 776.032 immunity - that's effectively a legal impossibility anyway, as if the case goes to trial, the state has to prove beyond a reasonable doubt that Zimmerman did NOT act in justified self defense; and that is harder to prove that fining it more likely than not that Zimmerman DID act in justified self defense.
IN any other case, I’d expect the judge to dismiss after the arraignment phase. In this one, he will probably let it go to trial with strict controls on what the prosecution can put on. IOW, kick it to a jury with little chance of a guilty finding. Of course in these days, that does not guarantee a not guilty given there may be a hung jury.
Then a refile on a lower charge if the vote is close, if not, no file...and Holder comes riding to the rescue with the case presented in Tampa JD with the sons of Obama on the jury.