I'm sorry you are going through this again, I cannot emagine what you are going through.
From my understanding Interbreeding can cause issues, if you have two dogs with an issue and you breed from them then it can affect the pups, I think of it as concentrating an issue.
I asked so many questions of owners, breeders and people who show when starting my quest for a pup, one answer sticks in my mind " you can have the healthiest parents and still get a sickly pup"
Bea has a very low COI (inbreeding coefficient) but she has a very rare condition, this said I would always look for a pup with a low COI, if you have the correct paperwork for the pup then you can find this my registering with MyKC online.
You may never get the answers that you are looking for, but I hope we on COL can help you through this difficult time.
Sharon