I've been at work all day so I haven't had a chance to read thru the whole thread so forgive me if it was mentioned already.
Being afraid of heights, the entire idea of an evacuation sounds extremely unnerving to me but if the situation became so dire I would suck it up and do what's necessary. But now having a 2yo son that raises even more questions for me. Being a 2yo he's very rambunctious and I could not imagine the ordeal it would be to navigate the top of a monorail, slide down the front if the monorail, then walk a track beam to safety.
Unless my life was in immediate danger I think I would wait for the fire department rescue ladder or cherry picker.
I believe (although this is an assumption, based on the pictures I have seen and my experience doing a car-to-car evac drill), that any Guests who transferred via the roof did so after RCFD had arrive and secured the Guests to the rail running on top of the trains, which is there for that specific reason.
Looking at the pictures and knowing that section of track, much of the train was on a curve which is surrounded by spur beams on either side. Most likely the scissor lift truck wasn't able to reach those particular cars due to the beams being in its way. I would guess what they did is position the scissor lift at the first car, which was easily accessible, then unloaded the Guests from that car, and then brought any Guests in other cars to that car via the roof hatches (again, after being secured), to then have them take the lift down.
Again, this is just speculation, but with some previous experience behind it. In addition, the Guests aren't actually up on the roof in the middle of the cars. What they do is take the Guest out of the hatch at the end of the car, they crawl over the gap between the cars, then immediately down the nearby hatch of the next car, then the Guests walk through the interior of the car, over the center bench, to the hatch at the far end, and repeat that procedure until they are in the desired car. You aren't walking the length of the Monorail on the roof. Most of the transfer is done within the car interior, with brief portions outside, to bridge the gap between cars.