In this line you've struck a false note: "He was still very pale and did not have strength to do more than complain about his plight."
I would avoid narration here, and go with something more like "He was very pale. A fine sheen of sweat made his forehead gleam in the lamplight."
"He did not have strength..." involves the narrator giving you a value judgment about Stuart's condition. "A fine sheen of sweat" merely tells you what's there, and lets the reader judge his condition accordingly: The singing is as strenuous a task as he can manage.
As I said, a quibble. The difference may be entirely stylistic.
I only want to be a better writer, so I welcome all advice.
I can already see an improvement in the novel from the helpful hints I have absord here.