Besides the story of Narayanrao's friend, are there other examples mentioned of people being cured of scorpion stings through Sai Baba's grace?

Yes, Chapter 33 provides a footnote with two additional accounts of healing from scorpion stings. In one case, Bapusaheb Jog was stung and went to see Baba. As he was climbing the steps, Baba simply said, "It will be alright, go!" and Bapusaheb's pain vanished completely before he even left the compound. In another instance, a Jyotishi Buwa who was stung in Rahata applied Udi given by Baba and continuously chanted His name. The pain stopped, which firmly established his faith in Maharaj. These stories highlight the various ways Baba's grace manifested to relieve suffering.