I don't believe she is FORCING you to return him, I think she is going ahead with what you said you wanted......to give him back.

I think you need to decide what to do.

The breeder is correct in saying that knowing an EXACT cause may not be needed in order to provide quality of life. YES, it is helpful to know the cause, but it may not affect the outcome.

Since you say this was your secod vet opinion, it may not help to find yet a third vet, but having a vet who can be open, honest and straight forward is an asset.