I went through this testing with my little guy. Here's a good resource to explain it: http://www.petdiabetes.org/cushings.htm
It took a couple of months for my vets to figure out if we were dealing with Cushings, diabetes, or both (because they are so similar and often mimic each other on tests). The 8-hour test you mentioned was the only way we could figure out what it was (diabetes) and treat it properly.
I dropped my little guy off in the morning and picked him up in the evening. They wanted to keep him all day because of the amount of tests and they needed to control food, water, measure urine output, etc. I can't imagine a vet wanting you to bring him back and forth all day.
I don't know how to tell you when you know if they have suffered enough. My little guy was in and out of the ICU at the vet hospital 6 times in about 5 weeks. His little heart just couldn't handle it anymore and I knew it was time. I understand how frightening it is to worry about their meds - I faced the same thing with giving my guy insulin injections twice a day. Not enough and he'll just get sicker. Too much and I might kill him. Had to make sure he ate first or I couldn't give him insulin and I had to cook the food because the only thing I could get him to eat was cooked chicken breasts. I spent quite a bit of time carrying him up and down stairs; ttaking him outside in blizzards; and cleaning up a lot of vomit, poop, and urine. But what choice did I have, he was my best friend. It was difficult!
I am so sorry that both off you are going through this. I hope that you get some answers soon.
There is an excellent Yahoo discussion group called Canine Cushings & Autoimmune. You may want to join that group so you can read through the information. They are an excellent resource.
I was recently in a similar position with my dog who had chronic kidney failure. Had the condition been a painful one, it would have made our decision much easier. As it was, we were able to keep her pretty comfortable for the 3 months she had left after diagnosis of end-stage renal failure. I won't lie to you. It was a fair amount of work keeping up with her special diet and meds several times a day, but definitely worth the effort. Even though we had to euthanize her in the end, we have zero regrets over how we treated her. Because Chica had her wits about her that last day (in spite of being barely able to move) we got oral sedatives from our vet before we brought her in for the last time. Having her sedated before leaving the house was a huge help for not only Chica, but for us. It's such a personal decision, so the only way I can advise you is to make sure you will have no regrets. Do what you can to keep your dog as comfortable as possible. Most dogs make it clear when they're done fighting; others like Chica, don't.
As for the Cushings tests, am I to understand your vet expects you bring your dog back and forth during the day? Why not just leave him off in the morning and pick him up at the end of the day? I know some people have had great success in treating their dogs with Cushings disease, so please don't give up just yet. The testing really is necessary to determine exactly what treatment will be most effective.
Cushings is really hard on the kidneys so that's why Gabe needs to go out more often. Once the underlying condition is treated, that necessity should decrease a great deal. I know it's scary to have to be the sole provider of nursing care, but really, what else can you do? Ultimately, as pet owners we truly are responsible for not only their lives, but their deaths. If you haven't already done so, post your question to Dr. Cheng on the free Pet Health board here at Med Help. On the right side of the screen under Related Expert Forums, click on Pet Health to access the forum. Good Luck!