As far as the Dietician, I think they work with the DR to get you to buy Vitamins, shakes and other stuff. Don't do it becuase they will suck you in like AMWAY. You only need to shrink your liver the two weeks before the operation! All the products you need are available at Taget, costco, GNC, Whole Foods, your grovery store and many more.
I would consult with a/your NUT on this for sure. She or he will help you though the entire journey if they are good! So you know, the two week pre op diet is not about calories or carbs ( you will get Protein during this time from shakes).. So don't worry when you get to that point. The little secret about Insurance and WLS is very interesting. The reason they want you to start on a 6 month diet plan (which you only need to do this 2 weeks prior for medical reasons) is because by that time, the insurance company has paid for the procedure with your premiums. Or better yet, they will hope you are so successful you won't want the surgery and they can save even more money. Insurance companies are only about the Benjamin's.. They suck.
In any case, do what you need to get the surgery! It is so worth it! Keep usupdated! Best wishes.