If I may suggest something? The saddle looks very narrow. Maybe it was put to one side because it no longer fitted anything (so you might have the same issues). One argument at the moment regarding the survival of small-sized antique horse equipment (I have a 4 3/4" cavalry horse curb bit from the 1850s) is not so much that horses (and riders) were smaller than us but that small sized items were used less and didn't get worn out, or were more likely to be left in store.
Just a thought
PS - The funny-colour seat is likely just where damp's got in. Shouldn't cause a problem and it would patina-out over time if you were to use it.
I'm pretty sure this saddle was stored poorly for a very long time, and probably got both damp/hot/cold/who knows what else. That's good to hear that the color might patina and even out. Nothing buffs out saddle leather better than a heavy rear spending hours in the saddle