It is risky to buy the M fare and waitlist for a mileage upgrade... as mentioned you have to clear before day of departure so the ticket can be reissued. Technically the mile upgrades don't transfer tot he gate, however some have reported being able to do so by contacting/calling up and asking to be activated on the list. Don't count on this though.

Not sure why you say that you'd be below everyone on the list... you would be below any DMs waitlisted for a GUC or mile upgrade due to status, but would be ranked with other PMs (if any) on the list.

Chances of clearing would all depend on the flight loads and when you are traveling. It is definitely more difficult since they down-gauged that route form a 747 to 777.