Gentlemen,
Before this important discussion goes the way of most on Dunnunda, there is one inescapeable fact.
The CAR/CAOs and company ops manuals make absolutely no mention of 'sky marshalls'. The legislation, as it stands, clearly lays out the responsibilities and powers of the pilot in command and also of the subordinate crew members. It also is quite specific about dangerous goods,be they loaded glocks or capsicum spray.
There has been no advice to the contrary, so why suddenly is this behaviour legal without any change to the rules by which we are compelled to abide by.
If a flight attendant in the cabin were to notify me that he/she believed that one of the passengers was armed, I would have no alternative but to declare an emergency and get the aircraft on the deck ASAP. How could you react any other way? You must assume, unless notified prior, that this person would pose a serious threat to the safety of the passengers, crew and aircraft.
Do to anything else would be incredibly foolish.