You mentioned that you had gone to a dermatologist, but have you seen a regular doctor to discuss your concerns about being underweight? It's important to remember that not all people are going to fall into the average weight- if you are eating a healthy diet (which your doctor can give you resources to confirm), and you don't have other health concerns, don't stress about the scale number.
As far as the acne- there is a product that I know has worked for some when nothing else has- it's from Rodan & Fields- I believe it's called Unblemish. That might be something that's worth a try. Stress can also cause acne, do you think that might be a contributing factor?
I don't have an answer for you. But just want to let you know that you are not alone. I am 42 and have been struggling with cystic acnes for the past 9 years (adult onset). The only thing I know for sure is that it is related to my GI issues. I had rectal cancer, GERD, gastritis, small bowel bacteria overgrowth, and precancerous lesions in my stomach. Symptom wise, I have always had constipation, nausea, stomach pain, early satiety, and significant loss of appetite with rapid weight loss and occasional vomiting recently. Whenever my symptoms flare up, the acne gets worse. It's due to the toxin accrued in your GI system. Usually after a severe episode of diarrhea and vomiting, when I went on a period of water fasting to allow my body recover, my acnes got noticeably better. Then, my GI symptoms would gradually build up again and my acnes would got worse too. They just follow cycles like this. If you haven't already, I would recommend that you ask your primary doctor for a GI referral and go from there. Good luck!
You can try raw African black soap or activated charcoal soap. Most people I know like the raw soap as it can be used for many skin conditions and also as a shampoo bar.