I just have a couple things to add.
First - and trying not to sound like a jerk here - I do feel that the store is justified. As most others above have said, your returns are excessive.
Second - I would spend some time over the next 2-3 months working on your knees and back, even at the expense of your normal training. Most running shoes are basically the same; the fact that you have a hard time finding one that works is less about the shoes and more about muscle imbalances in your body. I don't think it will take much cross-training to fix these issues. The key for you is to realize that you do indeed have issues, i.e. that it is not normal to have such a hard time finding shoes.
Finally, you should buy very neutral shoes and get used to them (possibly once you've sorted out your knees and back). Basic shoes with few features are what you need.