Did you know that the pup was missing a testicle before you received him here in the UK though? From your post, it sounded like you only realised this when you got him here in the UK.
I think the missing testicle issue is a totally separate issue to the other health issues you've mentioned and what ultimately happened, though. If you accepted him here with a missing testicle for 3 months, then it seems you accepted that situation and you can't go back to the breeder and say 'and I want more money back because he has a missing testicle' - because you accepted this. If, the first time you saw the puppy, you immediately contacted the breeder when you learnt he had a missing testicle and demanded a refund or replacement or something... then I think you'd be perfectly entitled to something if you had previously made it clear you wanted the pup for breeding.
As for what happened: On the one hand, you say he had a grade 2 murmur. But as others have said, that by itself doesn't really explain his symptoms. Instead, you had him PTS for some undiagnosed condition (albeit on veterinary advice). Personally, I would have wanted to know the reason and would have asked for a referral to a specialist centre where he could have a diagnosis made (as long as he was stable and not suffering and this could be done quickly before it happened again). I wouldn't have just had him PTS on the hunch of an ER vet with an undiagnosed problem, just because I've had many vets' 'hunches' be completely wrong in the past and ended up being happy I didn't go with the recommended course.
As a courtesy to the breeder, I would request his medical notes from your vet and send them on to her, so she can read for herself what happened and see that it was done on vet advice. I'd also give permission to the vet to speak directly with the breeder so the vet can answer her questions and explain why they advised PTS. Hopefully this will take you as someone to blame, out of things.
She cannot 'make' you get an autopsy done at a specified organisation - but if she is paying for it, why not?? I'd agree to the autopsy but she would need to pay for it, because it's her who wants it done. It may be important for her future breedings, to understand what happened to this pup and investigate whether it is congenital. This sounds like a responsible breeder who cares, to me.
I don't think there's any reason to be talking about courts and costs and threats - why are you even using that language, in having these discussions with her? She has lost a puppy that she has bred and placed overseas a long way from her, and you have lost a puppy that you thought would be a companion for a long time. You have both lost something, and now you need to work together to see how to proceed in a way that satisfies you both - not be talking of courts and threats!