I really don't think that higher protein feeds would affect a sway back or prevent it. Horses in general don't need the rich diets they get, in fact, the overfeeding probably contributes more to swayback than lack of any type of protein. Perhaps the feed helps fill in the crevaces with fat, making it appear less swayed,but you can't really fix true lordosis. Some exercises to develop the muscles in the back and abdomen may help, but once it's drooping, droop it does.
Poor saddle fit and obesity (and yes, carrying foals) can do it, but I do think genetics play a major role. Certain breeds are much more likely to develop swayback than others, regardless or feed, workload or having babies.
We have an old Appaloosa gelding with a sway back, and we have to carefully and creatively pad him for the saddle to not "bridge" and cause pressure points on his loin and withers. I actually sewed a pad myself for him to customize the fit. He rides fine, though he has other signs of the previous hard work he did in his younger years (we bought him a year and a half ago) and his legs are showing wear and tear, too. He's starting to get good weight on him now, and his back doesn't appear as swayed, but it is just as much as before. I won't ever let him get fat, though, it's just not good for ANY horse to carry extra weight like that.