Hey Gavin! First its good to know what your goals are. What exactly are you trying to accomplish. You said you are under weight? So are you trying to build mass?
1) It's personal preference although, I wonder if your eating fruit and staying below 100g are you eating any veggies? How much chocolate? If you're not have negative effects to it, go for it. Might be easier to just tell us what you eat in a day.
2) Everything within reason. How much is a lot? Are you eating like 90% fat? You need to be eating other stuff too. I would eat moderate fat and eat more in favor of protein and veggies.
3) How much protein you eat depends on if you are trying to do. I'm 120lbs I eat about 60-80g of protein on a given day. I don't think I could eat 120gs. That's just too much for someone my size. I believe that its 1g of protein for every lb of lean muscle mass.
4) You have to be a little more specific. Anything from feeling sleepy to weak bones, mood problems, muscle cramps. If you are worried about not getting enough or having too much go see your doctor. Do you take a multi vitamin?
Starting weight: 180lbs
Current weight: 120lbs
Goal weight: 115lbs