I think
@Clematis has said something very important to you,
@cchall. Here you are, excited about your upcoming surgery, date set, jumped through all the ridiculous hoops, and then a bunch of strangers come along and tell you something you very much don't want to hear, and something that conflicts with what your surgeon (that you really liked!) has told you. But facts are facts, and all the long term studies about the DS are based on a common channel of 100 cm or thereabouts, and the rate of nutritional problems is remarkably low, and almost always related to noncompliance. There are no studies showing excellent long term results with a common channel of 300 cm. None.
The nutritional deficiencies, or lack thereof, are almost entirely in your hands. It's possible that, in the patient population this surgeon works with, he has a higher than usual percentage of noncompliant patients. And even just a couple of difficult patients can color a doctor's opinion because they do stand out in you mind and you don't forget them. But that's them, not you. You need to do what's best for you, not for someone else.
I hope you'll look at this not as a setback, not as something to be upset about, but as an opportunity to reconsider what's been planned for you and make very, very sure you are doing what's in your best interest, and not settling for something that may be necessary to keep someone less well informed out of trouble. You are smart, you've researched, and you are someone who can keep up with protein and vitamins and labs. This is something you will live with for the rest of your life. Make sure it's something that will serve you well for the rest of your life.