I haven't read the source comics, which might explain it a little better, but I think as presented in the movie it's just that one day it happens. You stop being immortal - your body stops healing. Andy clearly lost her giveafux long before the start of this movie but she loses her immortality sometime between the ambush and Nile stabbing her.

It's also not clear whether they lose their immortality all at once or over time. Andy does heal, and she survives being shot by Booker, but doesn't heal up instantly anymore. It may be that she's losing it slowly, or it may be a temporary thing.

Quynh (I had to look up the spelling) is Andy's former partner/lover. The one who was taken and entombed in the iron coffin under the sea. There's dialog in the movie about how long the team searched for her and couldn't find her. The question of "what's her deal" would presumably be the theme of the next movie. She finds Booker (the subtitles tell us) six months after he's exiled from the group. We don't know how or why, yet.