I only see the seat move automatically (with the last seat position activated to door opening) when i switch keys and my keys have different memory settings on them. So i assume that when the iDrive menu reads "last seat pos" it means last seat position with that particular key, not the absolute last memorized seat position (ie, M1).
So on that note, unfortunately the X can't tell the difference between you changing the setting and the valet doing it. To go back you must press the M1 or let your valet use the second key to park the car for you. Or tell him to press the M1 for you maybe??
Now if I can figure out what happens when you put both keys in your pocket (with comfort access of course) and say hello to the X...lol.. just a thought.