First of all, I am SO sorry about your loss. I am also sorry about the way you were treated. As for the vet saying they are not at fault, maybe not legally, but I can tell you that a back to back treatment like she got is not really recommended any more. Some vets still do it, but it is terribly rough on these pups.
Nothing that can be done now, but I would probably be looking around for another vet if this one is doing such things. Is he AAHA accredited? If not, again, I would start shopping for one that is. My vet gives four months of Heartgard...then one injection...and the other is one month later. That is the recommended protocol now! There are other things done, but that is just a quick explanation.
Here is a link with information about heartworms and treatment:
American Heartworm Society | Canine Guidelines
Hope was lucky to have had someone in her life that tried to help her in the best way she knew.....poor baby...it is so sad that she even got heartworms. A cheap monthly dose of heartworm preventative would have prevented it. I am so sorry.