One of the reasons for compulsory deplaning is an immigation scam which became popular about ten years ago. Asylum seekers would swap seats during a layover with facilitators who had legitimate visas and who left the aircraft at the stop. I recall it became a big problem on one BA route in the Middle East.
Preclearance at Edinburgh seems very unlikely. It is a service provided not in order to advantage an airport or airline, but to reduce the burden on Immigration in US and to prevent ‘undesirables’ from getting to US soil. If 75% of EDI’s US traffic is US passport holders, it makes it a bit pointless, particularly in winter months. US passport holders may find it convenient but that is not the point of preclearance.