I would argue that Grace, especially the book!Grace, is not a coward, even before the mission. The "dang it, that's just not cool!" reaction to the revelation that he is a guinea pig doesn't scream "coward" to me. The willingness to barge into the lab and yell at Stratt to let him back on the project doesn't scream "coward" to me. The readiness to leave his life behind and work on the astrophage, even though he knows that any mistake might literally vaporize him along with the whole ship, doesn't scream "coward" to me.
He is not afraid to confront people. He is not afraid to act and fight and experiment, even if it puts him in danger. He doesn't "avoid risk like the plague", despite what Stratt says.
He is afraid of being sent on a mission where no amount of fighting will save his life. He is afraid of being served a death sentence. He is afraid of being doomed, being completely out of control.
That doesn't scream "coward" to me. That is just human.
