Matt - whatever she is, she is
, and I am sure that taking her back is not an option
BUT, I do think that to do the best you can for her, you should try to find out her true pedigree
A Beagle has a totally different temperament to a cocker spaniel, and if you did your research and chose a cocker for their breed traits as they best fitted into your family situation, then a beagle (or any other breed) are not going to live up to those expections.
If you find out what she is, then you can research her "real" breeds, and find out what challenges you are likely to face; you can meet other dogs of the same breed and talk to trainers and owners about how the breed are best handled
You might also want to consider reporting the breeder to Trading Standards to ensure that this is not a common practice