here is another way of looking at it and it might freak some people out.
Since it was ruined by water use water to fix it.
Wet the saddle again so you can reshape it to the way you want it to look.
After it looks right let it dry for a long time. tie it if you have to.
Push the bum dents back out at the same time.
After it dries treat the saddle with Proofide and let it sit a while before riding again.
Leather is a natural thing it can be reshaped. Do you remember getting a new baseball mitt ?
You would treat the leather put a baseball in it, but the glove between your matress and box spring and a day or so later you the mitt had a pocket for the ball. Or am I the only one who did that ?
Tony