Chapter 50 says Hemadpant is just a 'nominal cause.' Does this imply that the exclusion of certain saints was a divine choice made by Sai, not an omission by the author?

That is a profound interpretation based on the text's own philosophy. Chapter 50 indeed positions Hemadpant as an instrument and states, 'In this, there is no one other than Sai who makes the words speak.' From this viewpoint, the contents of the Satcharitra, including what is present and what is absent, are considered divinely guided for the 'true welfare of the devotees.' The ultimate aim is to achieve 'non-dual devotion' and a 'natural state' where distinctions dissolve. The text encourages readers to hold humility and perceive Sai in their hearts, suggesting the spiritual fruitfulness of the book lies in embracing its message as given, which leads to the realization of the supreme goal.