I am certainly no expert, but I see your concern with it sitting so low over the withers. My mare has really WIDE shoulders (not so much a wide flat back, that's more normal-shaped, but her shoulder blades are far away from her barrel and need a lot of clearance especially when she moves), and I used a western saddle on her for a while that looked a lot like yours, and even touched her withers when I put it on her without a pad. What I did to make it work in the interim until I got a better-fitting saddle was use a cutback pad with some built-up padding around the wither cutback area. This lifted it up off her withers, and the fact there wasn't even any pad where her withers were ensured she wasn't getting too much pressure there, and the width of the saddle (it was a draft saddle) gave her shoulders room to move.
However, what I ended up with is an SR Enduro saddle that has a much smaller footprint (while still distributing my ample weight well), and sits back further than most westerns do. I bought it used, so it was custom-built for a different horse, but it snuggles right into her back like it was made for her, and sits just behind the traveling range of her shoulder blade.
It looks like the swayed back is going to be your issue where the shoulders were my issue, so it's definitely a whole different thing and my solution won't be your solution, but I'd definitely be on the lookout for saddles that are made to fit the shape your horse is, in general. Ask around for brands that tend to fit sway backs, or go for a fully custom tree if you can. And in the meantime, do what you can to both get the saddle off the withers, and see if shimming in the center will provide more even pressure--the problem with such a curvy back is that all your weight may be resting on a few small points at the front and back of the saddle, rather than the saddle evenly distributing your weight. Good luck!