I ride switch with 15,-9. It doesn't matter what angle you ride, even foward angles. Its all about practice, practice, practice until it kicks in. Took me about 2 seasons before switch riding clicked in my muscle memory.
Sure, you can, but it would suck. I used to ride a 15/-6 but switched to 15/-12 and then finally to 15/-15 because switch riding sucked with 15/-6. I don't ride switch a whole lot, but it's nice to have a setup that makes it easier when I do. I didn't really notice any negatives when changing it, it actually helped my knees quite a bit.