Having just had my first broody ever, who just hatched her first chick this is what I've learned and will do different next time...
I went through the same thing you are ... all the hens wanted the box that the broody picked to nest in. Not only did I have multiple hens in on top of her laying their eggs but their non-fertile eggs were getting under the broody. I had marked the fertile ones at the beginning so I was able to continually pick out the non-fertile ones to bring in for breakfast.
Next time I will move my broody to a special area once she's been sitting for a day or two. If she's truly broody she will continue to sit on her eggs in the new spot. It's much easier (I found out) to move her at the beginning than to wait until after the chick(s) hatch. Once the first chick hatched I didn't want her in that nest box (it was two feet off the floor and I didn't want the chick to fall out), but... I didn't want to move her and the eggs (and chick) during that critical last 24 - 48 hours of hatching.
So, move her now to a safe place where the others cannot get to her, where she can't get off the nest to eat and then go back to the wrong nest box. Mine did that multiple times leaving the eggs to get cold. I was actually surprised that I even got one chick. It was the only one I got from the five eggs. I do think it's because the eggs got cold several times. I had to go out multiple times every day to make sure she was sitting on the right nest box.