Okay, that was just stupid. She's still breathing, and then 2 seconds later dead? I mean...what the hell was that?
Unfortunately, that is painfully realistic: people just don't fall over dead immediately, but some of their bodily functions try to keep going as long as they can, so even victims with lethal injuries breathe for a bit even while there is literally nothing anyone can do for them. That's why triage exists in the first place. But I do agree with you that, apart from extra realism, it is unclear what the author wanted to achieve with this plot point.
I also have a feeling that these last few chapters are extremely rushed, which probably means the manga is getting the axe and Ogino Jun is desperately trying to cram the rest of the story in the few chapters he has left. This one chapter essentially contains three episodes: Yorino's rescue and loss of an eye; Youko's death and Yorino's breakdown, with the reveal of Manabe's double in the background; and finally the attack on the Bastion and Yorino's return to action.