You can't save Duck no matter what you do. Carley/Doug dies no matter what you do. Major plot points play out one way or another usually regardless of your decisions, although they often play out in slightly different ways.
You can't save Duck no matter what you do, same with Carley/Doug depending on who you saved in chapter 1.
I don't mean JUST those two, I mean everybody. And I completely forgot about Doug.
I'm pretty sure all major character deaths occur one way or another regardless of your decisions. Your choices do affect later dialogue options, as well as how characters view you later on. The game gives the "illusion" of choice, while you do somewhat have control, most major events happen no matter what.