Well what is done is done and all you can do at this point is hope for the best. I'm wondering what the original vet thought about the mammary cancer. That alone is very serious and should have been treated immediately when you brought her home last year. When are you going to see the new vet? You really need to ask this vet about the cancer. It could easily spread and since you did not have her treated since last year it could be life threatening.
The breeder that you purchased this girl from is beyond irresponsible that she allowed one of her dog to be rehomed without being spayed. It should have been the very first thing that she did before rehoming her. Accidents do happen but the real blame for this goes to the breeder for her irresponsible practice of not spaying prior to rehoming her.
Please let us know how she is doing. Make sure that you keep that vet appointment and when the puppies are born please let us know how momma and babies are doing.