"First Edinburgh airport is about to get easier to get to for NE travellers, next year the first tram route opens from Princess Street straight to the airport. This effectively means you could get off the train at Edinburgh cross the road and get the tram to the airport." - have you attempted to find a vacant parking space at any of the key East Coast Stations in NE England recently (and if you find a space, being able to afford the car parking fee)? A fairly strong disincentive to the rail option, even before lugging baggage from Waverley up the hill to Princess Street, followed by a crowded long tram ride.
"Finally the airlines would have to pass on the savings with the scrapping of APD. If you read the draft policy document produced by a future Independent Scotland you'll find that the new government will fine airlines that don't! "
Well it will be interesting to see how an independent Scottish admin survives if they effectively start placing caps on the profit margin that a business can generate.
"Evidence of this already exists remember the easyjet expansion went into reverse at Newcastle when they opened a competing base at Edinburgh."
EZY expansion went into reverse when Jet2 set up camp in NCL, and provided competition, and arguably that's why EDI then became a more profitable base.