I would suggest the use of a man riding cage (first aid cage), if you have a piece of lifting equipment to lift it with. Failing that perhaps via a window opening on the building via the scaffold so as to not have to use the ladder access, as for the roof I'm not sure.........

Might struggle to maintain three points of contact. ;-)

This post really highlights why ladders on scaffolds should become a thing of the past except in exceptional circumstances. Stairways are so readily available that I can see no good reason not to use them. With proper planning they can be readily achieved. Look at the hierarchy of risk control and think about rescuer risks

Look at the mountain and cave rescue equipment they have some exceeedingly good rescue stretchers you just need the competent persons to use them:-)

How many people are trained to move casualties? I ask because it is not a part of First Aid training and moving a person with a broken limb, internal injuries can cause significant further injuries. I would advise that where a person has fallen a significant distance and has recieved injuries that a first aider would have difficulties diagnosing accurately leave them where they are and call the emergency services to help you move them. After all having got them off the scaffold wrenching their broken limbs around and jiggiling about their internal injuries are you going to dump them in a car and rush them to A&E or are you going to call an ambulance. Call an ambulance first leaving moving injured people to the professional where you can and only move a significantly injured person as a last resort.