Is it really the case EDI management could not accommodate a single flagship long haul service on a legacy platform that entered service in the 1980s?
Skipness One Echo; I think the simple answer to your question is probably yes; they would have struggled to accommodate the DL B763 any earlier back then.
There are four stands at the international side that can take a standard B752. Stands 2, 3, and 4 are contact stands whilst stand 104 is on the opposite side of the cul-de-sac and can only be used when stand 103 is not in use.
AFAIK, only two of these stands (2 and 4) can accept a B752W or a B763 and none can accept a B763W because the stands are too narrow. As more and more airlines fitted winglets to their B752s, it became increasingly difficult to accommodate B752Ws and B763s at EDI so on too many occasions the DL B763 had to wait for a stand to become available. Passengers and aircraft turnaround were then further delayed on too many occasions because of the ridiculous corridor arrangements between stand 4 and the immigration hall which mean that passengers cannot disembark from an aircraft on stand 4 if passengers are boarding an aircraft on stand 3.
The situation is now even worse as many airlines are fitting winglets to their B763s. B763Ws are too wide for stands 2 and 4 so they need to use the SE apron stands but this can only happen very occasionally due to the severe PCN restrictions on Taxiways Lima and Mike and the consequent CAP168 inspection requirements.
BAA's priority at EDI in recent years has been to improve the shops, restaurants and security rather than to fix the PCNs, sort the corridor arrangement from stands 3 and 4 to the immigration hall, make more large stands available and make a variety of other improvements that might make the airport more attractive to long-haul. I completely understand why BAA took that approach and I'm quite certain that it made very good business sense for them as a monopoly provider of commercial airport facilities in the Scotland and London, but it has done nothing to help attract further long-haul routes to EDI.
I’d like to think that the new owner will see the potential for long-haul from EDI and invest in the necessary facilities but there is no guarantee that this will happen. We’ll just have to wait and see.