In what ways can a RN convince potential employers that he/she is fully capable of fulfilling duties as a bedside nurse after working in administration for 25 years?

The best way is to show that you've kept up with advancement in nursing. For example, CEUs and/or another degree completed. You will still need to complete a skills lab no matter what. That's a small price to pay for going from administration to the floor. Nurses are needed, so it's a good time to go back