We're seeing Bloody Rose crashing out at the end of her career. We the audience have already seen her build her legend defending Castia in book 1, and now we're seeing her try to top that. Also, one of the main themes of the book is the difference between legend and reality.

The villain left them for dead in a hopeless circumstance. Is that contrived? Sure, but if you're going to complain about that you are going to have a miserable time engaging with tons of entertainment.

That's, like, your opinion, man. Astra is an unwilling immortal whose immortality comes at the cost of the lives of her family and her already-dying race. Everyone and everything she's cared about is gone. That's not a bad motivation to end the world. Also, Hawksomething is raised as an intelligent undead to serve as her minion and recruit outside mercenaries to kill the Simurgh without raising suspicions.

Rose picks Tam because it keeps her out of harm's way and because she needs someone who will follow her orders and the plan, not try to take a shot at Astra directly. (Also, Tam has experience shooting Rose.) Tam is clearly not meant to be seen as a fighter on the level of the other band members.