Is Ginko's death a foregone conclusion (in the Mushishi storyline)?
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 mushi that colonizes the host from inside, seeing as it came out from Nui's second eye to devour her from outside, and the same happened to the one-eyed fish which Yoki saw in the pond during daybreak.

However, there are cases where people were able to avoid death.

In episode 20, Ginko went to meet Tanyuu Karibusa, who happened to have a Mushi inside her, but she was able to slowly "get rid of it" by writing the stories of Mushi's being killed. The ink used was the caged Mushi, which thereby got sealed into the writings.

In episode 9, Ginko met a head priest of a village who happened to use a seed having Kouki (the life pulse of all the Mushis) inside of it. The head priest was dead by the end, but Ginko revived him by injecting the same seed into priest's mouth, and then, he became immortal. Though such is a forbidden practice, it is always possible.

Based on these three episodes, I'm completely certain that he can live as long as he want. He is no ordinary man who can be engulfed by Tokoyami easily. I'm certain that Ginko, being a Mushi-Master, can find a way to cure himself.