After reading the thread, I found myself wondering how steep your driveway must be, and have you had this problem anywhere else besides at home on your presumably very steep driveway?
I would say you disproved the "slipping clutch" diagnosis with your mark on the serpentine belt. No need to pursue that avenue any farther. Obviously the clutch is holding well enough to turn the engine over through the tires.
I would guess there is something wrong with the parking brake. Most vehicles will roll on a very steep incline if the parking brake is not holding well.
As for the compression loss theory, you can get a compression test done pretty cheaply or buy a tester yourself at harbor freight for $20. My guess is your compression is fine. It would be worth a few dollars to get the test done to eliminate that as a possible issue though.