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)?
Doesn't that get in the way while sleeping? Don't you ever knock it down?
What I would do is connect it the opposite way round. So that it folds down, not up. Give it a little leg that folds out when you pull it up.
The leg could be done in a few ways. But I would prefer it so that it is connected to the table top's underside on a hinge. So, when the table is folded down the leg is tucked behind it, and when you pull the table up, the leg "drops" down and upright.