"You have either better, or worse, eyes than me as I can't see any sliding doors in the first video clip. Of course that could be because the coach had already been converted to open plan seating some years before."
I watched on a large screen, the sliding doors of the compartments are definitely visible. It's probably the Brake Executive Generator Van. It's definitely a generator coach because it has the side venetian vents. The last coach in that consist is an ex BR BFK Standard Class Generator Van, again you can tell be the side venetian vents and lack of a rear window for the driver. I would imagine that 2 generators were required for this long 13 piece consist. If you continue watching the video, the next train is another loco hauled Enterprise (remember that it's a GAA specials day). That train is only a 6 piece but the first coach is a ex BR BFK Standard Class Generator Van and the final is a Brake Driving Trailer.
"The first coach in the second clip is indeed the Brake Executive Generator Van, but the second coach is in fact 813 now numbered 917, the original Driving Brake First from 1970. This vehicle would have had a large Guards Compartment as described by JHB. "
As you mention, the second coach is the original driving brake first. The last coach is a driving brake second....you can see the drivers window at the back. I would imagine that the Brake Executive Generator Van was added to the consist (and probably locked out of use) purely to be used as a generator.