Julian,

Yes, I was in the sea scouts (waits patiently for laughter to subside from those that picture me in a sailor uniform) I also come from a family of six generations of seafarers (my father was the master of a 160K ton ship until he retired) and I suspect I have forgotten more knots than you have ever heard of, how about a reverse, square sennet?
Knots dont work in Kevlar, due to the elongation characteristics of aramid fibres, its the same reason that they are so good at dissipating enrgy, it also has a very low coefficient of friction, typically 0.03 and knots rely entirely on friction to hold.
As to the steel arm, steel does not like shock loading, if one component has suffered catasrophic failure what hapens if it goes into a violent spin as it leaves at high speed and puts the rigid tether into a bending moment against a corner on the robot? Or jams under commpression inside the robot?