The author, Yuki Urushibara, does include authors notes at the end of each volume of the manga, explaining what inspired each story.
About "Heaven's Thread" she wrote:
"This is a story I thought of when reflecting on old folk tales my grandmother told me.Tales about how missing people were left upon a huge rock. There are similar folk tales all over, ...
Given the variety of events that happened in first season, it is safer to conclude that a Great Mushi-Master can live as long as he want. Though I agree with your view that as per Nui, anyone who lost one of its eyes to Tokoyami is bound to become Tokoyami sooner or later.
As far as I understand from the death of Nui in episode 12, Tokoyami is more like a ...
Tsuzurahara Miyu, apparently.
Incidentally, AnimeNewsNetwork tends to have very comprehensive listings of voice actors and staff, so that's always a good place to check if you have a question like this in the future.