Hi Range,
Welcome to the board. I'm probably not the best person to answer your questions, but since no one else has posted a reply, I thought I'd give it my best shot. I would have to guess that dwarves in minis is more common than dwarfism in AQHA. However, careful breeding practices, knowledge of bloodlines and pedigrees, and avoiding breeding stock with dwarfy characteristics are helpful in reducing the chance of getting a dwarf. Knock on wood, I am one away from completing my 10th foaling year and have yet to have a dwarf born here. I'm sure part is due to luck, but I think part is due to watching in-breeding/cross breeding and not using animals with dwarf characteristics. As for locked patella, I had a gelding who suffered from it for about a year following an injury when he attempted to jump out of a 3" high pipe gate stall at a horse show. He gimped around on it for what seemed like a career ending forever, but eventually it went away completely and he went on to be a National Champion in performance classes. I have another mare who has problems with this if I keep her stalled up. I don't know if she had an injury before I got her, but she has it in both back legs. I've not had the problem with any of my other horses. I don't think it is that prevalent and I do think there is a simple surgery to correct this if you do get a horse that has it. Probably one of the bigger things that people don't seem to discuss as much in minis is a retained testicle. I think this is a bigger problem than many breeders want to admit to and with the way AMHA rules are written, a World Champion junior stallion can go on to be a BIG NAME stallion but could never be shown as a senior stallion due to only having one testicle.
Hope this helps.