Get to the other stuff first and see if you can get that working.
The leather is easy.
First conservative application of thin leather dye with a brush.
Then neatsfoot oil. Again, not too much. It can weaken glue. Apply with a soft toothbrush, to get into every crevice. Wipe it off after an hour or so with a soft cloth.
Gluing lifted leather should contrary to common wisdom, probably be done after oiling and drying. Don't put oil on the surfaces to be glued.
Contact rubber glue can easily become a nightmare, though it is in theory "the correct thing" to use. Using padded rubber bands to clamp down soft rubber erasers, with plastic bag material underneath, on tradition paper or wood glue, is probably better overall.