Much sympathy. I'm in the same boat, more or less, except I'm almost done and I just got diagnosed. And no mixed episodes yet, as far as I know. My meds are being changed every other week, which makes dissertation writing extra hard. Like it needed any help...

Don't all grad students abuse caffeine? Just kidding - it's such a stereotype, one just assumes it's normal!

Coursework is busywork in many ways. The only thing to be done about it is to get it done, and get on with it. Life gets much better once the coursework is done. Fewer constraints on your time and more flexibility to work the way you need to. If you're not being challenged in your courses, though, talk to your advisor and/or profs - I'm sure they can help you find ways to make it live up more closely to your expectations! I always figured that going the academic route means that if I'm bored, it's my fault. That means it's also my opportunity to fix it - what a privilege! Most people's jobs don't allow that freedom.

Most of the time, people who leave a PhD program never come back. It's not that they couldn't, but life moves on. Making a real paycheck for awhile makes it harder to go back to student wages. It's hard to transition back into the academic world if you've been away from it. After a long enough period of time, credits expire, etc. You can probably take a leave of absence if you need to (check with your program director) but if you can hack it and still have the passion for it, IMHO you should keep going.

I have found my advisor (and most of our faculty) to generally be quite sensitive to the "real life" needs of PhD students. This is not a sprint, it's a marathon, and you need to keep up your health. I've had to dial back my work hours and travel considerably because I was getting pretty burned out, plus all the bipolar stuff popping up. I can't work 80 hours a week (for long) without starting to have serious consequences, so I'm just not going to do that to myself. No one has complained yet.

Good luck - it's a tough spot to be in. I hope it improves soon!