Take the dog to another vet. Every dog needs to be tested for heartworm before starting a heartworm preventive program. The only time that a heart worm test is not required is when you have faithfully given the dog it's heartworm preventive year round and even then the vet will have you sign a release.
As for a puppy holding his or her bladder for 8 hours I would have asked her if she can hold her bladder for 8 hours. She probably would have said no and then I would have said then what makes you think a puppy can. Do babies hold there bladder for 8 hours?.
Ask friends and relatives what vet they use and if they are happy with them. If they are then go to that vet. If this vet was recommended to you that gave you the WRONG answers then tell the person that recommended that vet to find another vet also.