Hi.
While I don't have any experience with the S60R, my 99 V70R AWD has never had a feathering problem. Not only is it AWD, but I have used both Bridgestone and Goodyear max performance summer tires, with only short tire life to show for it. Exactly what you might expect, nothing more.
The Volvo factory alignment specs are probably not the issue. The alignment can be within spec, but if the right and left wheels are on opposite edges of the specification it some times can cause problems. Unless the car keeps going out of spec, the dealer is probably just wasting their time and yours by repeatedly aligning the car. In addition, at least from my experience, once a tire is worn like that, they pretty much stay that way regardless of whatever corrective action you might have taken. I'm curious about your driving style as well. Fairly low speed turns with a lot of powered understeer can definitely feather the front tires. As I remember, the Haldex AWD system still sends most of the power the front wheels.
You may consider taking advantage of the tire warranty. Obviously you have tried to correct any mechanical problem with the car. You may be able to get the tires replaced if there is the possibility of a defect. Try a Google search under the manufacturer and model of your tire + "defect" or "defective" and see what comes up.

Good luck,

Mark