Was Amir Shakkar's rheumatism treated with conventional medicine by Sai Baba? If not, what was the remedy?

No, Sai Baba did not use any conventional medicine to treat Amir Shakkar's rheumatism. As Chapter 22 explains, Baba's remedy was simply His word and command. When Amir prayed for deliverance from his ailment, Baba instructed him to stay steadily in the dilapidated Chavadi. The text explicitly states, "Baba's medicine was only His word." Despite the externally contrary conditions of dampness and cold, Amir's healing was contingent on his internal faith. By accepting Baba's word as the medicine and staying in the Chavadi for nine months, his "certain faith" ensured that everything went well internally.