I read that Amir was banned from the Masjid. So how exactly did he get to see Sai Baba so regularly?

Although Amir Shakkar was forbidden from visiting the Masjid, Chapter 22 explains that the Chavadi's location provided him with frequent, effortless visions of Sai Baba. There were several ways this happened. First, Baba's daily morning route for begging alms passed by the Chavadi, allowing Amir to see Him as He went and returned. Second, every evening, Baba would come in front of the Chavadi and salute the directions before returning to the Masjid. Finally, on the alternate days when Baba slept in the Chavadi, there was only a nominal curtain and a plank door between them, and they would talk privately after the other devotees left.