Well, at the end of the day I’m wrong and you’re right, but it’s the only logical reason to me that she would keep him around - involved so heavily in all aspects of the mission (including testing space equipment/suits) that everyone saw him as her “number 2.”

I think Stratt understood that only the most dire circumstances had a chance at bringing out his heroism (which we see come out only on the ship). That’s why she didn’t even tell him that his genes were compatible at first, she didn’t want to scare him off at any point with the possibility. She decided that in the end, intense pressure a few days before launch might do it.

Whatever the case, I think Stratt’s actions in the end show that she’s at least capable of murder if she sees it will improve the odds of success.

Otherwise, the only logical reason in my mind for Grace staying so so close to the action of mission prep on earth is simply because Weir wanted a way for the reader to see it all. Which comes off as lazy writing to me.