As to the legal liability, that is easily solved. Beyond the scope of this forum other than to day that's why we have lawyers and peer review.
And the stark reality is that at the end of the day, you just don't know what the spring constant is without testing the particular piece of bungee...
And having made thousands of load/elongation curves, no, they are not strictly linear. Hookes law more or less works in part of the range however, and this makes it useful.
Interesting. A LOT of incorrect or not-quite-correct info in that thread
All rubber used in tubes or as bungee (both natural and synthetic) is vulcanized, typically in a Banbury mixer.
That rubber is then put through a milling process, where it is rolled between two or more steel rollers. This...
So, I am writing a textbook on working with bungee in the theatrical, dance and circus industries. This book is focused mostly on the "these are the knots you use and how to tie them; this is how to splice bungee; this is how to make a bungee pack; this is how to put on a bungee harness; this...