Limp and Heart Problems make me think of lyme disease. My girl's advancing murmur was basically halted when she was put on antibiotics, though we had no proof that her heart problems were caused by infection. I wonder if the breeder's dogs have ever had lyme - they can be passed on to the pup, and it's not clear if it would easily show up in testing.
My girl's grade 4 murmur was VERY loud because of the positioning of how the blood was being pumped. It made it sound very very bad. The cardiologist had me monitor her respiratory rate, said if it was ever about 40, to get her in ASAP. It was usually around 12-18. She typically had a very low heart rate though.
It really sounds like you need a new vet. I would also have a cardio consult.