One of my favorite YA novels of all-time. I thought it was entirely realistic. And the fact that the author got me to cry at the end, even though I already *knew* the aftermath of everything, was like... wow.
I was once a suicidal teen, and I'm now married to someone who has regular bouts of suicidal depression (and his father committed suicide; and I could go on forever about people I knew who offed themselves, most of them teens). Every case is different. I honestly don't think you can have something NOT be plausible when it comes to wanting to kill yourself. There are so many factors that contribute to it, and no one ever sees all of those factors from an outside view no matter how well you know the person. How can someone reasonably say, "I don't think someone would really do that"?
Takes a lot of balls to suggest that you know what someone would or wouldn't do in a suicidal mindset, when a good majority of the time, the person who is suicidal doesn't even fully know what they're capable of doing or what they might try. Just saying.
My thoughts on the MC in 13RW are simple. She had already committed to the act of killing herself, in her own way and at her own specified time. The tapes were her way of making her peace with the world, so to speak, before she left it for good-- not necessarily meant to explain or justify her suicide. How she went about that and why she went about that are not things you can really question. And I think a lot of people who focus on the plausibility of her actions are missing the point of the novel entirely-- the message of hope at the end, despite all the bad that happened.