Hi, @julesb
I'm not sure that Warwick trussrods are any different in effectiveness or reliability that any other commercial offering for two way rods out there. And the advantage of using a Warwick one in a Warwick is that you absolutely know they are going to fit. Over the years, I have fitted pretty much every rod maker's products and quite a few of them need slightly different chiselling because of subtle differences in their dimensions. That is fine if you are fitting them pre-fretboard gluing...but more of an issue if one gets stuck halfway through inserting them in the Warwick system.
Ref 'multiple failure', then I think the clue is in your observation that the presently failed one is upside down. In my earlier ventures, I came across a similar reversed one in what should have been a 'standard' bass (ie not Rickenbacker/Gillett, etc that do have reverse actions) - and it surprised me how much bend I'd inflicted before I twigged that the gap was, indeed, getting bigger!
So, if a previous owner who maybe wasn't familiar with trussrods had replaced the original broken one upside down, and then had just followed the instructions of 'righty tighty', it's entirely possible that it would have weakened it by the time that it was realised that it needed to work the other way.
Hope this helps
Andy