I would apply a treatment to the leather sparingly just where the leather folds over the edges of the bellows, outer folds and inner ones. Sparingly each time, but not just once.
Neatsfoot oil/Neats-foot-oil. A neat is old English for a cow or ox.
Coacholine. A proprietary leather conditioner brand.
Furniture shops sell other leather conditioners.
Dry leather is more likely to split.
I don't know if you know how free-reed instruments work, so here goes.
The only connection between the two ends is the bellows, and if the leather has dried, it may be fighting you.
Each button or key opens a flap which allows air to flow through a hole and through a reed, making the note. If you are only holding one key down, then the effort you are making will seem harder than if you are depressing several, so make sure that every note is sounding, whichever way you are working the bellows, in or out. Also that each right-hand note is the same volume.