You do realize that should an incident occur in the cabin your FFDO FO has the authority to tell you where you are going to land your aircraft?
Do you realize that ultimately, the Captain's authority rules in the air and that he will inform you of what his plans are?
This argument has no winner. Any complaint you have on the ground for where a Captain chose to land following such an event will not have much negative repercussion for the Captain unless you can prove some manner of negligence.
So... exactly how many airline Captains do you think are out there that believe they must obey their armed co-pilots orders?