It's interesting that The flying gunman - as a serving officer in (I assume) DRPG rather than SO19 favours the 'overt' rather than 'covert' approach.
That approach works well with the average nutter or drunk (read air rage passenger) where the sight of an obvious CPO (usually wearing the de rigeur shades) is enough to put them off any thoughts they may have of getting a few minutes of fame.
However, that approach - in my view - would be counter productive in the case of a 'professional' attack. Identifiable security officers would be neutralised first; and it is significant that both the US Skymarshals and those used on El Al are in civvies.
Weapons to be utilised would also be a critical choice - they must be powerful enough to put the target down yet at the same time not overpenetrate and hit innocent pax. Pancake rounds or rounds like the Glaser or HydraShok would be the most effective; and indeed so would be Tasers although care would need to be taken with them on 'electric' aircraft.
I agree that a minimum team of three would be required (which coincidentally is the minimum number needed for a potentially successful hijack) who would need to be spread throughout the aircraft. The costs involved - at say £50k pa each for the right calibre of people, which is senior FO pay on a sizeable airline and Captain pay (or more) on smaller ones - would be prohibitive, especially as one team would be required per crew; and the loss of revenue resulting from the occupation of three seats would also have to be factored in.
Under present rules, in the UK the only people that would be permitted to provide an armed escort service would be police officers; and to source say 7,500 qualified officers (500 aircraft x 5 sets of a 3 person team) would be completely impossible!
So that leaves you with civilian officers who would have to be unarmed under existing rules. They might be fit and well trained - but so would be the people they are going up against. And unless full body searches are introduced, the chances of detecting composite or ceramic weapons will be minimal - as is the case with certain types of explosives. Profiling helps, but is far from infalible - and again, the pros know exactly what is being looked for and will act accordingly.
The most difficult thing to counteract is the sleeper - someone who gets employment with a specific company or industry and works perfectly normally for an extended period until they are 'activated' and carry out required tasks. These sleepers could well include pilots.
Today's terrorist is usually highly intelligent, well resourced and funded - and represents a clear and present danger to us all.