I have no idea how the judge would rule, but I do know this: The judge would want a great deal more information before coming to a conclusion - including the captain’s side of the story. (As Rainboe has sensibly been urging.)
A judge reading this thread would be amazed at the way some posters condemn the captain on so little information, especially without knowing his side of the story, and surprised that those who have never operated an airliner express an opinion at all. (Unless the judge had been reading PPRuNe for years and had seen it happen many times before.)
“The judge may have never seen a cockpit from the inside or even flown.”
True, and would probably have no expertise in the operation of airliners. That's why the parties would probably call expert evidence from people who have. ie Experienced airline captains.
Neither side’s lawyers (unless they were spectacularly incompetent) would attach any weight to the opinion of someone who wasn’t and never had been a professional airline pilot and based his/her opinion upon being a frequent traveller, or having managerial experience in some other field or holding a PPL.
The underlying question would be pretty straightforward: Was there a threat - and if so, was it justified to wait for 5 hours to divert because the decision was so difficult to make?
So it all boils down to the factual scenario - what did acutally happen on board of that aircraft? First come the facts, than the expert witnesses who assess the facts as provided by the witnesses. So there will most likely be a plethora of witnesses (including independent witnesses with no affiliation to either side) who will describe it in the way it has been described by BoF in this thread (totally true or only partially true, whatever - the sheer numbers will not be in favour of the captain). The CA will tell his side of the story as well, and I am pretty sure that every judge will take into consideration that he is the most senior professional on board and that he has the authority to take whatever action is needed to guarantee the safe operation of the aircraft, i.e. address any real threat or reasonably assume that a certain situation is threatening even if it turns out after review that it was not. However, I doubt that a judge will take the view like many on this thread that the definition what facts amount to a threat for the safe operation of the aircraft is COMPLETELY up to the captain when a review takes place. The captain will have the utmost discretion to decide this, but there are clearly some limits - evidently so if the airline takes a wholly different view than the captain. What next? No sugar with the coffee is a threat just because the captain says so? In short, of course there will be expert witnesses, but they do not assess a theoretical scenario, but the facts as they are presented by the court after hearing all witnesses. And as far as expert witnesses are concerned in a case such as this - United will easily find a Dani-style expert witness, the captain a Rainboe-style expert witness. In the end it is up to the judge to assess as a layman what expert evidence thrown at him is more convincing to him. Expert witnesses enable the judge to make a somewhat learned decision, but if there is, as so often, conflicting expert evidence, not much is won from the judge's point of view. Unless he tosses a coin, as a human being faced with such a dilemma he will apply common sense and a test reasonableness as long as the burden of proof and evidence presented to hin allows it.
Just in case if oyu are wondering, I am no layman as far as law is concerned, quite to the contrary. The theoretical application of law is one thing, how it works out in the court-room is another. You will know that as much as I do. Based on an assessment what the most likely evidence presented to a court would be, I would be very very hesitant to tell the captain that his case is rock solid and that he has nothing to worry about. So how about a golden hand-shake?