I think the key here is taking the time to learn to ride switch properly. There is no better way to train the other side of your body, then to actually ride that way. I would certainly duck out your back foot to at least -6, possibly more if you are going to make a big effort to ride switch. Don't be afraid to drop the angle of your front foot either. You are at the point where you need to experiment with your stance to see what works for what. Make small changes, but try a variety of stances. If you don't try, you will never know...
No, there's nothing wrong with riding in a forward stance. A duck stance is more versatile however, for the typical riding you see at resorts. It gives you more options in the way you can ride your board. Personally I also find it a lot more comfortable. There's nothing saying you can't ride a twin board setback either. That said, in my opinion a true twin snowboard performs at its best with a centred and ducked stance.
Have a read of this article on
switch riding. Also if you haven't already, read the article on
stance.
Let me know if you have any questions.