A used saddle is always an unknown. If the leather has dried out, the fibres have broken and the leather will tear with use.
No treatment can remedy that.
After the Proofide treatment, and a little use you find it uncomfortable, then you can accelerate the forming of the leather to your sit bones. Just apply a dampened cloth to the top. leave it for about 30 minutes until the leather is damp. Then go for a ride. The pressure from your sit bones will mould the leather to your shape. Spend about 30 min on the saddle, with frequent checks to see if you are making an impression. Once you have the shape you are after, leave the saddle to dry over a few days without any further use.
A thin coat of Proofide and see how that goes.
Wet moulding vegetable tanned leather is how the saddle shape is formed. You are just using the leather tanning characteristics to form the leather to your shape.
Last weekend, I cut a new leather cover and then immersed the leather for about 30 min in a bucket of water and then I moulded the wet leather into the desired shape. It is a very narrow Sprinter width which I am making into a Swallow style.