I read your posts and you seem to be really focused on attaching the bag to the passenger backrest. I think you'd be surprised how little wind hits back there behind you and the windshield.
IMHO...The bottom four metal D rings should allow attachment DOWN to the luggage rack. If you "at a minimum" attach the two forward most corners of the bag to the front of the luggage rack, the bags not going anywhere. Adding the backrest strap really just keep the bag from slipping around a few inches in all directions.
These bags comes with tie down straps but I've changed to Velcro straps that loop back on themselves. Even found some with metal clips that I attach to the D rings then wrap the Velcro around the attachment point on the bike.