Your failure investigator said that the shafts failed due to "fretting corrosion", which is corrosive wear and/or cracking caused by relative motion between two tight-fitting surfaces subjected to cyclic, relative motion of very small amplitudes. Most splines fits are designed for ease of assembly. They generally are not interference fits. Some movement between the male and female members is to be expected. If these splines are "dry", not lubricated with a grease fitting or gearcase oil, and in a corrosive atmosphere, they will corrode. Since the corrosion products are weak and brittle the slight movement between the mating parts will continue to expose fresh metal. If the splines are located were you can't get to them to grease them, look for a replacement gearbox that features "wet" splines.