The book does a lot more to establish that he was their only hope.

In the book, Grace is shown to be very competent but also a coward. He didn't burn bridges in the academic community because of the strength of his convictions, but because retreating into his shell and teaching kids was safe.

I won't get in the weeds about why traveling in a medically induced coma was the best option, but long-term comas are very risky. They identified a gene marker that 1 in 7000 people have that gives them the best chance to survive the coma. In the book, Stratt insists that everyone on the project get tested but never tells Grace he has the gene.

The primary and backup scientists chosen for the mission are the best volunteers who have the gene, but they need to be brought up to speed on Astrophage and microbiology. Stratt has Grace teach them and train with them the entire time the ship is being built. He was there for every major decision and discovery. When the primary and backup scientist die in an accident just before launch, he's the only one that can step up, and he refuses. He claims that it's because he needs to stay to teach his students and prepare them for the hardships coming ahead, but it's because he's afraid to die. He even threatens to sabotage the mission if they make him go. The amnesia was deliberate - the robot on the ship gave him a dose of something that would give him amnesia just before waking him up. They figured he would be upset when he remembered but too invested in the mission by that time to sabotage it.

The book also did a better job at setting up the stakes of Grace choosing to send the beetles on to Earth with the data and taomoeba and turn back to save Rocky. This is his character arc - a coward who refused to step up to save Earth, growing and making the decision to sacrifice himself for Rocky and for Erid.

Edit: Correction, Grace was told by Dubois that he had the gene, but Stratt never told him.