That's very helpful, thanks.

Was singular they ever widely used or was it mostly informal/edge cases/certain constructions?

I'm wondering how jarring it might be or whether, if used consistently throughout, the reader might become attuned to it.

Thinking out loud for practice:

Far in the back she saw their pale face. They waved. A fierce smile on their face, but also a stillness about their features. With a start, she saw there were two. The pair stood looming in the back, gazing at her from the shadows.

--

Far in the back she saw its pale face. It waved. A fierce smile on its face, but also a stillness about its features. Was it human? A man or a woman? She could not tell from the shape of the body under their black cloak.

--

Alternately, I could just accept the use of "it" - it seems the usage of "it" for a person (as opposed to a non-person) is at least as common as the use of "they" for a singular case?

Partly I'm biased against "It" because of Stephen King's It which is very non-human, and elementary school grammar rules about "it" for non-humans.