Sorry to hear about Duke's diagnosis. Glad you were able to develop a plan quickly.
As for not having dogs on HW preventative, where we live it's quite common. I have to specifically ask for a HW test every year at the vet. All of our jokes about it being a dry heat aside, it is very dry and not really much risk of HW. My dogs are on a preventative because we travel with them, and going more than 50-ish miles from home can put us into a climate where the risk is present. If they were homebodies I wouldn't do it.