Not having read all of the posts:
I dismount by kicking both feet out of the saddle and launching myself off. But I’ve also dismounted in the traditional way by weighting the left leg and swinging the right leg over the saddle. It’s the same pressure as when you mount because riders don’t leap on their horses.
If my horse backed up when I dismounted, I’d swing my leg over to dismount but would stay in the saddle and then swing it back and correct the horse if he backed up. Or get right on again if he backed up. Maybe do a little bit of work. I’d do this until the horse understood that backing up in not an option and it is easier for him to stand still and harder in the grand scheme of things for him to back up while you are dismounting (that means more work for him).
If he doesn’t back up when you mount but does when you dismount, given what you posted, I think it’s a behavioral thing.
Make the right thing easy, the wrong thing more difficult. Horses get it.