Hi
You sound like you have researched very well
If you are happy with the health tests carried out, have visited the breeder and happy with the situation, her set up and her dogs, she has satisfied all your questions and you hers, then I would go with what you want.
My Reese has show champions on his mother's side and on his father's mothers side but on his father's male line a few generations back is a working cocker. Reese is 2.2% I had researched the breed and wanted the healthiest happiest cocker I could find, i speont a long time looking for a breeder and found a lovely breeder who cares deeply for her dogs, She spent a couple of years trying to find and buy the right stud dog for her lines and was very happy with his confirmation, health and temperament, so she had thought very carefully about trying to improve her future puppies.
I was so very sad to read the post from geordietyke and the problems she had with her 2 previous well bred puppies, so who really is to say that introducing one working strain dog so far back has/hasn't made improvements. I think it's down to personal taste and what you want your puppy for.
I hope you find what you are looking for soon and look forward to lots of pictures. :luv
Take care
Tracy & Reese