Prior knee surgery, current knee pain, and maybe one leg shorter is dangerous to experiment with. By the way flat feet is really over pronation and inserts or arch supports usually help that. I would take it easy and not try anything radical until you save up for some experienced help. Ask around and find someone that knows cycling anad knows how to treat your issue - it could be a bike fitter, a sports MD, or a podiatrist that sees you on the bike.