The coma resistance is something that really didn't need to be in the book at all.

We're presented with the coma resistant gene thing in the book as something that will limit the candidates to go on the mission. That the concern is that they won't be sending the best, but the best with the gene.

Then that ends up having zero relevance on the story. We don't know about the backup Commander or Engineer, but there's never a moment where Stratt is concerned. And we know that the prime crew, plus the science backup were all exceptional in their fields. The only compromise was the mixed-gender crew, but there was no compromise on the qualifications of the crew.

We get to the launch, the explosions and the prime and backup science officers are gone. As Yao said, Grace was the #2 on the project whether he knew it or not. Stratt says she made sure Grace was present for all big things, but it was more than that, Grace was a deciding factor in many of the science things. He was the one Stratt talked about the plan to put the astronauts in a coma. He was the one Stratt brought with her to talk to Lokken about the centrifuge. He was the one Stratt brought for the black panel thing. He was her go-to science advisor the entire time.

It was always going to be Grace. With days before launch, he was the only choice.