Air Canada just took another B789 (from BHX no less!) and had added a whole subfleet of ex SQ A330s, the pain point was B767 sized aircraft with the wholesale removal of that type from mainline and latterly Rouge. Two more B789s to come (only one?) before the A321NEOs.
EDI's proposal to allow passengers free reign in the main terminal post any US CBP formalities surely won't work, not least from a Customs perspective, you can't freely mix with the hoi poloi then arrive at a normal US domestic gate surely.