How often do you rotate your tires, I believe 5-8 K is recommended.
Just checking you guys out from time to time and came across your thread on rear tire wear. My 2008 2LT malibu seems to have the same issue with it's rear passenger tire wearing on it's outside edge (feathering). all tires are inflated properly and i rotate them every 3000 miles. the car has just under 17000 on it.
I suspect the alignment has been out from day one from the factory. I plan on having a lifetime four wheel alignment done on it very soon and recommend you have it checked and done as well.