An intact bitch is a big deal and I'm rather shocked your breeder says it will be okay. You ask 100 breeders if their bitches got along after a fight and I'll bet you at least 80 say no. The problem is even worse because they disrupt the entire pack, and you can end up with fights due to the tension with the other dogs. I lived 7 years with a bitch that I didn't spay until she got her championship at 2. She was incredibly dog aggressive and the only dog she didn't try to fight was my male Rottweiler. Every other spayed dog in my house were targets. It is hell and nothing I would choose to ever go through again. (She died with cancer, btw. ) I have never loved a dog as much as her, but it wasn't fair to all the other dogs. The lack of stress after she died left me with a lot of guilt over not rehoming her.
I'm also perplexed that you have to keep the bitch intact, but not required to show in case she wants to breed her? If your contract stipulates you cannot spay her, however hard it might be, you may need to consider sending her back before it damages all peace with your current dogs.