Bunnylady--luxation of the patella is DISLOCATION of the patella--entirely a different thing from locking stifle, which is UPWARD FIXATION of the patella.
The conformation that leads to locking stifle is VERY hereditary. Your excuses are very typical of Miniature horse breeders, and go a long way in explaining why locking stifle is so common in Minis.
It's true that an injury can cause locking stifle--but in truth it's a very specific type of injury that is most likely to cause a problem with locking. And, if the stifle is actually injured it is going to be very obvious. If you don't see swelling and/or feel heat in the stifle, and if the horse isn't extremely lame/stiff/sore (not to be confused with simply "locked up") then chances are that no injury occurred. I have had two horses with injured stifles; both were perfectly obvious that the injury had occurred, and neither one ever locked up.
Congenital luxation of the patella is rare in horses with the exception of minis and shetlands. I suspect that there isn't a lot of "proof" of what causes the patellar groove to be too shallow (which is what causes the luxation in newborn foals)--people can guess at the degree of heredity in this condition, whether it is a combination of genes from both parents, or if it can come from only one parent, or if it is simply some sort of mutation...but I sure wouldn't not write it off as not genetic.