They should have free access to hay, and then a certain amount of pellets depending on season, age, activity, size etc. You can give different vegetables/fruits, but it's not necessary, and you should not give too much as they can be quite sensitive. Leafy vegetables are good though, you can give alot of that, with a few exceptions. Never do any quick changes in their diet and introduce new food slowly, even if it's something healthy and good for rabbits.
#2 It's mainly for vitamins and minerals, yes, but I'm not sure what they are made of exactly. I don't have much knowledge of the subject tbh, but I'm guessing the ingredients can differ widely between brands. I will do a lot more research if I ever decide to get new rabbits. (Haven't had any for years).