Hello,
I don't think there is anything that can be said to make you feel better about the loss of your beloved pet since even if you knew what it was that ultimately caused her death, it was so fast that you had no time to even accept it. My heart goes out to you.
I do think that Addison's is a possibility as even for vets, it is sometimes missed. It is often referred to as "the great imitator". Many dogs that are affected have signs that wax and wane as well, so the owner (and their vets) think they are getting better an then they crash again. It is possible her cortisol levels were low due to the Lysodren and the stress of everything in the past week was too much for her. A normal dog's system could handle that, but if they have no way of producing even a normal amount of cortisone, they are at a loss. One thing that doesn't make sense however is that she did receive Temaril -P which has steroid in in so it should have helped her.
It is also possible that there was another disease going on. Kidney failure can have similar signs. I have also seen dogs with Cushings that had a large enough pituitary tumor that they had central nervous system signs like seizures. Unfortunately, you will never know.
What you have to focus on is the fact that while you had her, you gave her your love and the best care possible. I have many clients that wouldn't have even treated their Cushing's dog. You didn't give her that disease and once you found out she had it, you did the best you could to keep that disease from killing her, accepting the risks involved. Think of how terrible you would feel if she would have died from a blood clot to her lungs ( a life threatening complication in Cushing's dogs). I think she knew you were doing the best you could for her.
I am certain you will continue to have uneasy feelings, but hopefully with time, you will be able to look back on the years of great memories you have and not the way things ended. Please accept my condolences and let me know if you have any other questions.
Dog Veterinarian
Over 20 years experience treating dogs and cats