InOban, from my role at Glasgow & Edinburgh can vouch that during the English hols the Scottish airports are full of Geordies, Cumbrian, Liverpudlians & Mancunians. Remember the rail station built specifically for this to happen at Edinburgh. Glasgow will have it's rail connection as well.
Norwegian will go where the they get the best deal. MAG, BHX & AGS would have spoken to them. What ever was said, done or not offered will be for their respective owners. They all have to look at their current airlines, there's no reason to let Norwegian in only to see AA, DL/VS, UA or TCX leave. At the moment APD is the same. If the Scottish Government lower it then instead of complaining about the Scottish Government, take it up with Westminster.
All I'm saying Norwegian could yet prove a nasty pill at Edinburgh & maybe Manchester will be lucky as they'll still have the other carriers. It's very interesting time. Lots of seats to fill Transatlantic with a falling £. I know having recently been in the US, it's expensive over there.