They can be used for most things but not Prelim or novice dressage .
They should only ever be used by someone who understands how they work, how to fit them and someone who has an independent seat and can keep their hands still
Yes you could. In the past people hunted in doubles. But, personally imo, I think you would have to be a very accomplished rider to do it well, or very competent at riding correctly in a double bridle. There are so many other options in terms of bit nowadays Im not sure there would be many people who would opt for a double over other bits for x-country!