Sounds like you're on the right track. Have you asked your vet about medications that regulate PTH and calcitonin? Seems to me that no matter how much calcium the body takes in, nothing will happen if the right hormones aren't present to regulate uptake.
This may be far-fetched, but dairy cows can suffer from "milk fever" (very low blood calcium) during their first days of lactation. Treatment is a simple Ca injection, but if untreated, the cow can go comatose and even die. To counter this, they are fed rations with anionic salts in the days prior to calving. It tricks the body into increasing the blood Ca concentration, which offsets the drastic decrease at calving that normally causes milk fever. Can they do something similar in dogs?
My retriever had thyroid cancer. He had half of his thyroid removed. They told me he would live about a year and instead he lived another four and died of an unrelated cause.
Good luck to you and your girl - I know where you're coming from.