Same thing happened to me this morning. Logged on to check my SSX scores and updates, and all my gamerscore had gone. Got the points back for SSX (345 or so). I guess it checked the offline save games or something.
The way I see it, rewind should rewind you and every other racer. Just like any other racing game... Why it doesn't do this just confuses me to no end, and yes it's about "-6557 x20" useless in trick it mode too.