Joe is right, the leathers should slide through the opening in the tree and you should be able to rotate them enough to still use your keeper. And they will stay bent easier too, because you will be bending the leather where it is thinner instead of fighting the thickest part of the fender like you are now.
My main reason for posting though it to tell you NOT to wet the fenders if you are concerned about keeping them looking nice. I recently bought a like-new saddle for $250 which is a $1000 saddle. The only flaw with it is the previous owners must have really drenched the fenders to try to turn them. They succeeded in turning them but the fenders have the ugliest water marks on them now. Big dark, ugly water marks on an otherwise new light oil saddle. I could just kick them for that! But maybe I wouldn't have gotten it so cheap if they would have taken good care of it. But your saddle is similar in color to mine, so I wouldn't wet it if I were you.