Any suggestions for sensible knots that would allow me to a) tighten the loop around the screw at the bottom post-hoc and b) adjust the length of the cord post-hoc (but it holds the weight of the boards + books)?
Yeah a piece of wood the same thickness as the shelf on a hinge attached to the frame could swing out and support the shelf. When folded in it would be the same thickness as the shelf folded up.