The dominant foot should be the lead foot on a snowboard. If it isn't most likely the rider is riding and steering from the back of board which while it can be effective, isn't efficient.
Your dominant foot is the foot that you most feel comfortable balancing on. if you make contact with a soccer ball kicking it with your right foot,the it typically will be the left foot ( or regular). if you wakeboard, long board it usually the same stance. I would stay with the goofy stance since that is what you are comfortable with already long boarding. Don't worry about all the other tests. You already participate in a sideways sport so stay with that. Some people are so close/even it really doesn't matter. It would be a good idea to ride switch a lot. It will make you versatile.
That's weird you mention that. I am right foot dominant when it comes to things like kicking and such, but I longboard/snowboard regular. I am tempted to say I have pretty decent form as well with minimal to no rear foot steering.