So sorry about your Pom. That is a terrible thing to have happen.
One of the possible causes of Cushings is cancer, either of the adrenal glands or of the pituitary gland. If Bunny did have cancer, it would have been the cancer that killed her. You and her doctors successfully treated the symptoms of Cushings, but the cancer that caused the Cushings was still progressing. Please take satisfaction in knowing that you made her comfortable for her last couple of months of her life. Also please know that, if she did have cancer, there is nothing you could have done to prevent it. That type of cancer in dogs cannot be prevented, and it cannot be cured. You did everything that could be done for her, and you did it lovingly. You have cause for sadness, but you have no cause for regret. I wish everyone took care of their dogs like you took care of Bunny.
My 8 mth Pomeranian was doing fine she went missing for the night was back in the yard the next day and was barley moving wouldn't eat or drink she died that night
I don't have any answers for you, but I'm sorry Bunny died. Cushings is a horrible disease and hard to control, and its so hard to watch your dog live with it. You may never have an answer, but try to find some comfort in the fact she went on her own and you didn't have to make the decision for her. She was a lucky dog to have you try so hard for her and love her so much.