The 769s aren't planned. They have been mentioned, but there are no plans as yet.
Electric units will not be regularly timetabled to turn around at Dalmeny, until several years in the future, and even then it's very unlikely.
Electric units along the line, if they are pure EMUs, will be chucking people off at Gateway, turning around in the Up Passenger Loop, and leading back to Waverley. As was so astutely pointed out, there is currently exactly one electric starter from Haymarket heading east every day, and this is unlikely to be extended to more than one. Timekeeping would be atrocious, and the current timetables work well without trying to thread North Berwick services through Waverley to the west.
It would be more prudent to get back to the topic at hand, being the state and progress of mast foundation installations, and when they will start progressing along the line towards the bridge