@Ms M I am so sorry you have been through so much! It's sad that a simple desire to improve your health can lead to so much heartache.
I really don't know what to suggest. It does seem strange to me that you would have been diagnosed with bile reflux with a DS, because that would be almost impossible. With a DS, the duodenum is completely divided just a bit past the pylorus (the valve at the end of the stomach) and the new connection is far downstream. Bile would have to travel upstream quite a distance to get to your sleeve and from there to the esophagus, which is incredibly unlikely, if not outright impossible.
Even with a true gastric bypass, there is a RNY type connection of the small intestine to the pouch, or whatever part of the stomach it was connected to in your case, making bile reflux very unlikely in this case as well.
I have no idea how having gastroparesis would affect the function of your stoma either at its present size or a smaller size. I also don't know how having a narrower stoma would fix bile reflux, as bile is a very thin (usually) liquid and can easily flow through a narrow passage. but then, I also don't understand why you would have bile reflux to start with.
On the positive side, it sounds like you don't have to deal with the insurance issues we face in the USA. Apparently if your surgeon thinks you need this procedure, he can get it done. It sure isn't like that here! But, how he thinks it's going to benefit you is a mystery to me. I hope he finds a way to help you.