A lot of apartment complexes paint their units between tenants anyway. I would consider using some EZ anchors to mount to the wall and then just fill the hole with shrink free spackling when you move. You could even touch up the spot with some paint as I'm sure your apartment maintenance would be glad to let you have a bit.

Also, there should be a stud within a reasonable from your desired location. There is nothing wrong with moving the speakers over a bit and then just accommodating for the distance in the AVR. My Left and Right surrounds are not exactly the same distance from the center listening position because I wanted them mounted into studs.

The QS8's are very forgiving with placement so they don't have to be in the 'optimal' location to sound amazing.

-David