I'm sorry your dog is experiencing so many health problems. I understand your reluctance in putting her through any distress at this point.
Plucking the hair from the ear canal doesn't have to be a major truamatic event especially if you do just a little bit every couple of days. This is not a difficult thing to learn how to do. It is easies to do if you have a hemostat (this is surgical instrument) and I believe you can purchase these from pet stores. If not, your vet could certainly get you one. You can also use a needle nosed pliers if you want too. If you grab several hairs with the hemostat, twist it and pull, you can remove this hair yourself. If you do just a little at a time, dogs usually don't mind this very much. If you are not clear on exactly what I mean by my description, just ask your vet to demonstrate it once for you.
Controlling ear infections in dogs can be very challenging. Since your dog has Cushing's she is going to be more prone to developing ear and skin problems, so this is something you are likely to have to keep dealing with. Using a good ear cleaner (Malacetic Otic or EpiOtic) and using this twice a week or so will help keep the yeast population within the ear canal under control, but probably won't actually clear up the infection. You could also ask your vet about trying what we call "pulse therapy" for chronic infections where you treat the ears with a topical ointment one week out of every month on an ongoing basis. This helps keep the infection from getting out of hand.
Another treatment that worked for yeast infections for years is an old home remedy of mixing white vinegar with water in a 50/50 mix and using that as an ear cleaner. This (as well as the cleaners I mentioned above) help keep the ear canal at an acidic pH so it makes it hard for the yeast to survive.
I hope this information is helpful. If so, please click Accept. If you have additional questions on this topic, please let me know. Good luck!