Your right, this is not normal!Yikes! That sounds awful, I’m sorry that happened!
I don’t think your experience is common though. My mom’s Pullet was a fantastic mama hen and her chicks did fine. I had 3 first time adult broodies this summer and they all did pretty well. One chick was injured from hens fighting over it but it recovered (I learned to separate the new mamas from each other!). OP won’t have that problem since there’s only one broody.
Since I decided to give all the broodies at least a fair shot at it, it was all on my shoulders!
Anyway, I think the op should give it a go, as long as they are willing to accept whatever outcome.
I am sure most hens will be fine. Most of mine were just unfamiliar with where to set their weight, and where not to.
Unfortunately this didn’t bode well for the chicks that did hatch.
My broody's clutch should be hatching next weekend just in time for the nice weather to disappear! Its nerve wracking going into winter with my first broody hatch with a first time 10month old mama! Hope you're girl sits tight for you! 
