Ugh, I know that feeling. We just lost 3 people when we were already running a bare-bones crew at our 24-hour store (2 of them only gave a week's notice and then didn't even finish out their week! The other one got fired so on top of everything else we've had the Loss Prevention guy breathing down our neck). Thankfully I'm on overnights (11pm-7am normally) so I'm not stuck picking up all the open shifts like I probably would have been if I was still an evening shift supervisor, but now us overnights keep getting stuck with everything Evening Shift doesn't finish on top of all of our stuff that normally takes all night anyway, so that's been fun.

Maybe even just something small like "Well it's hard to get all of our stuff done when we start out having to finish up the previous shift's work" would help drive home the point about unrealistic expectations to your pushy manager, or at least remind her to also hassle the previous shift so things feel more even. Hope talking to your next-up manager does some good, or at least makes you feel better about the whole thing.