Anytime a disease or defect affects a breed or group of breeds more often it is known to have some genetic basis. Legge calve perthes most commonly affects toy breeds and small terriers. It is widely thought to have a genetic cause, however they don't know the mode of inheritance - ie is it autosomal recessive, multigenic, etc. They don't know what gene(s) cause it just like the currently don't know what genes cause liver shunt (but they do know it us genetic) if you breed two LCP dogs, there is a high mode of inheritance in the offspring. This is why breed clubs and the OFA recommend not breeding any LCP dog.
This article discusses some research behing LCP and it's mode of inheritance
SHOW DOG MAGAZINE, Leg Perthes in dodgs
So far as the contract, it I'd true that that is what you agreed on so legally she's under no obligation to help. I personally would never agree to such a contract. I would only sign a contract that agreed to pay medical costs up to the purchase price of the dog. It is a loophole to include the words "life threatening" as it is open to interpretation, and to only offer a replacement always gets the breeder off Scott free because no loving pet owner would return their dog. These contracts are so common and appear okay to many pet buyers, but they really don't offer much at all in my opinion. The breeder ought to pay medical cost up to the purchase price of the puppy.
Two other thoughts: why would a buyer want another puppy from a breeder when they already got one with a problem? And two - why would the breeder want to give another puppy to a buyer that would return a pet for being sick? A loving pet owner wouldn't give up their pet.
Now with alltgat said - having one puppy with a health issue does not in itself make brenda a bad breeder. It will show in how she handles it. If she's researched her lines and health screened, she has done all she could up until this point. You unfortunately can't identify carrier dogs right now, and it might not have showed up until these two particular carrier dogs were bred that produced the genes necessary to cause disease in this puppy. Hopefully she will do the right thing and stand behind her puppy and make sure the parents aren't bred again, as well as any offspring. Even the most careful breeder could get a puppy pop up with an issue - it's all in how they handle the situation.
To the OP, I'm sorry about your puppy