from my amateur knowledge 40m is where recreational divers usually stop as it is survivable if you spend only few minutes at that depth. but even then they always do several stops to keep it safe. so around 12-13 stories. Not sure how deep did she go.
Walker betraying them felt really out of character. They are probably setting up a redemption arc for her. But still was off, as her ex would never forgive her when learning what she did.
The timeline for those kids to be survivors feels a bit off. they would be babies or not even born when rebellion happened