Runway capacity isn't really that big a problem. Stansted think they can reach 51 million with a single runway, and Edinburgh could extend the runway if needs be.
Would they get planning permission for an extension though? Stansted's runway/taxiway system is far more modern and well designed too.
Preclearance doesn't drive demand, it just makes arriving in the US easier. People aren't going to schlep all the way to Edinburgh for the privilege of avoiding a queue which in my experience is usually in the region of 30 minutes (and frequently much shorter!)
It isn't really a schlep for large portions of this country though.
Large chunks of the North West have direct trains to Haymarket, then you can change to a direct tram (or the bus if you prefer) that is mostly reliable west of Haymarket.
Glasgow/west of Scotland also have plenty of direct services to Haymarket.
NE England/Yorkshire is a bit more complex, but connecting to the tram at Waverley or changing onto a Haymarket service isn't that much of a ballache.
The EDI airport experience is significantly preferable to Manchester (if Liverpool/Glasgow/Newcastle had year round flights to the US, they would be good alternatives - I wonder how many connect through Toronto in the winter if aiming for a northern US destination city).
Of course Heathrow, Gatwick or maybe even Birmingham would be preferable, but I don't think the former have the space or capacity, and Birmingham doesn't seem to attract many intercontinental flights, despite its excellent road and rail connections (especially with the coming of HS2).
Those passengers leave the UK from convenient airports like London Heathrow. Unless you're suggesting BA will stop off in Edinburgh on the way to JFK then this imagined massive increase in flights won't happen.
The ones I was talking about mostly leave regional airports like Birmingham/Manchester/Glasgow etc.
Most passengers from Heathrow fly direct, rather than connect in Dublin.
Only for people within easy reach of Edinburgh. Otherwise, if you are predisposed to have a much longer journey time to get pre-clearnace, why wouldn’t you go via Dublin and avoid the long haul APD?
APD is a point I hadn't considered to be fair, but if you live in parts of the Midlands, Yorkshire, North East, or North West, there isn't that much that difference travelling to Edinburgh vs travelling to Heathrow.
For that matter, why wouldn’t you just use MPC (Mobile Passport Control, which is free) at US immigration and fly direct, saving yourself at least three hours compared to flying to Edinburgh first and connecting? Last twice I’ve been to the US my wait time at immigration has been measured in seconds.
ICE based concerns!
It would obviously cause an increase, but I don't imagine it would be "massive". For example, I can't envisage passengers from London and the Southeast shifting their travel from Heathrow direct, or via-Dublin one-stops in preference to travelling to Edinburgh. And, in the event that there was a huge surge in demand for US flights, it would be perfectly reasonable to shift some of the lower-cost holiday departures to Glasgow, not to mention the unused capacity at Prestwick.
London + SE + Wales + East + SW is only about half the country's population though.
You could exclude half the Midlands and NI (for different reasons), but you still have quite a significant population willing to try pre clearance and not too far away.
Good to mention the unused capacity at Prestwick, perhaps Preclearance at Prestwick would be something to look at under a new US administration.
It already has a rail connection, plenty of runway capacity and the terminal could be upgraded with investment.
Road connections aren't too terrible, it would be a good opportunity for Ember to provide coach links if there was a reason to.
People have already mentioned the weather-based advantage too, and it has historical precedent as an airport providing transatlantic services from Scotland.