In my opinion, Marilyn's choice to stow away in the emergency ship was very believable. It is made clear in the story that she is pretty young, and she was unaware of the consequences of stowing away. She thinks her punishment will be something along the lines of paying a fine. As Barton says himself,  "She wanted to see her brother. She’s only a kid, and she didn’t know what she was really doing."

In terms of Marilyn's response to her fate, we can actually see her go through the five stages of grief: denial, anger, bargaining, depression and acceptance. She initially goes through denial, saying Barton must be joking; then she experiences anger, accusing Barton and the others on the cruiser of wanting her dead for no reason; later she bargains, asking Barton if there is any possibility another ship could save her; she experiences depression when she talks about wanting to live and never seeing her family again; she experiences something close to acceptance when she says goodbye to Gerry. Of course, there are other factors in her response, but I think with this progression we can see her response as believable.