I read that Amir Shakkar was forbidden from going to the Masjid. How, then, did he get to see Sai Baba?

It's a fascinating point mentioned in Chapter 22. Although Amir was forbidden from visiting the Masjid for Baba's vision, the very place he was assigned to—the Chavadi—ensured he received darshan automatically and effortlessly. He was able to see Baba every morning and evening. Baba's daily path for alms in the morning took him right past the Chavadi, allowing Amir to see him as he went and returned. Similarly, every evening, Baba would come in front of the Chavadi to salute the directions before heading back to the Masjid, giving Amir another opportunity for vision.