Accessibility tips for Devils Tower/Black Hills?

For info, I think you should check with the National Park Service on Devils Tower and Mount Rushmore, as they are not in the USFS domain. I'm not sure who has jurisdiction on Crazy Horse, so can't help you there. You won't have any problems at Mt. Rushmore as everything up there is ADA compliant; a wheeled walker might be helpful for DH to get around as there is a seat to sit on when he gets tired.