When our dog had Cushings, her back legs would go out. We carried her outside when she had to go, and carried her back in. She would improve after a day or two, and walk fine. She was on Trilostane, which I don't think much of. She had to go on Prednisone because the Cushings meds worked too well, but the Prednisone made her worse. Good ;luck in trying to find the right dosage and meds, its really hard.
Some dogs will survive Lymphoma, it all depends on how fast the cancer is growing. My first dog got this and he had a fast growing cancer and I went throught all of the cancer treatments and he lived 6 months longer. I had to make a finacial choice to allow him to go, it was hard to allow him to suffer, will it was only the last two days that he could not breath well when laying down. Ask your vet if there is any other medication to use for the cushings and also ask them to tell you how long they really feel that your dog will live, this will help you make the right choice. Prednisone will keep the cancer at bay for some types for a long time. I am sorry that your going thouth this, I do know how hard this is. Don't you suffer to long, it is very hard on us as well.