I don't think the framing/interpretation of Ryland Grace as a selfish coward is an *incorrect* reading of the text, but I do think it's kind of an uncharitable one.
More so in the movie than in the book (and I DO think the movie generally does a better job with the emotional beats of the story), the main impression I get from him is not fear of death (though he certainly doesn't want to die), but fear that he's not enough. He genuinely thinks that he's the wrong choice. He thinks he can't do it. He thinks that if they send him, when the stakes are so so high, that he'll fail and doom everyone on Earth. Which is, to me at least, a deeply understandable motivation.
Anyway, I think that if he'd been given more than 3 hours, if he'd really managed to internalize that he was the only one who could do it, that he would have agreed to go. Not happily, not bravely, but I really think he would have (and also that his whole thing with threatening to sabotage the mission in the book was 100% bluff). The fact that he didn't get that time is part of the tragedy of the whole situation (delicious, for the record).
But we see it pretty clearly in the parallel scene where he decides to go to Rocky at the end - he knows he's the only one who can possibly help, and he knows what to do. He very calmly accepts that if he does this, he will die. We can frame this as Grace finally finding someone he cares about more than his own life, but I just don't read it that way. He always had it in him. It's just different when he gets to figure that out on his own terms.
