In my saddle making days, I would have made duplicate pieces and replaced the ink stained ones. It is that difficult to remove ink.

I have heard, but not tried, that it is possible to remove ink from leather with dish washing detergent. As ink is a dye, I doubt this will do much. The ink gets into the pores of the leather and is very difficult to remove. You can try it, but if it doesn't do the trick, let the leather dry completely before trying something else. I never had any luck trying to wash ink out.

Do not use any kind of bleach. That will damage the leather. It will also remove the patina which is very hard to try to restore.

The best thing to try will be 90% Isopropyl Alcohol and a soft WHITE cloth. Colors can run and compound the problem. Saturate the cloth and work at the stain, turning the cloth to keep from smearing the ink. At best, you will minimize it some. The alcohol will dissolve the ink, but the ink is going to be carried farther into the leather.

Be sure to oil the leather with pure neetsfoot oil or olive oil after you give up. Do not use Neetsfoot compound as it will contribute towards thread rot. Any animal or vegetable oil is OK for the leather but certain oils like corn oil attract mice. Mice don't like olive oil and it is nice and light and will penetrate deeply.

While you are trying to remove the ink with dish washing liquid, scrub the whole thing with a soft bristled brush. Work up a good lather and rinse well. Once it is dry, oil it up and seal with a light coat of neutral shoe polish or good carnuba wax.