??? a) the cotter pin is there only to prevent the castle nut from loosening after it is fully torqued. There is no "load" on that cotter pin, under any normal circumstances, that I could ever think of. Heck,,for that matter there is no real load on that nut either..it's only purpose is to seat the taper and that taper is what all the working loads are transmitted through
b) if the castle nut "popped off" and the threads were stripped (??) then its highly unlikely that you can simply put a new one back on and expect it to stay there.
I guess what I'm getting at is that your "failure" is indicative of some other odd problem and very uncommon. ME..I'd very much want to get to the bottom of what is really going on there. Heck, I hardly ever even bother to install the cotter pins on those ball joints..