It's a myth that he painted lying down. I believe this comes from representation in the film, The Agony and the Ecstasy, before this movie no one ever thought of him as lying on his back.
Try lying on your back and controlling your arms, focusing in the near distance, it would be very very difficult
Michelangelo designed the scaffold himself, it was a wooden platform. As I understand it, and seems likely, a temporary device, light scaffolding would have been used to erect the platform, which was then accessed by ladder