Reword #2: When she is abducted by her estranged father so she can be spared, a stubborn humanitarian must find a way to prevent him from killing most of humanity.

I still think that a word or two about what she will be spared from needs to be added.

As mentioned in previous posts, how humanity will be killed is important. What is it she is trying to stop?
Her goal and actions are very important for the reader to understand the plot.

Agree with Richiev. ?The story needs to specify the method; it's not an incidental detail.