Hello!
Thank you for the query.
GERD symptoms and diarrhea can be caused by gastrine secreting tumor called gastrinoma. Symptoms caused by this tumor are called Zollinger-Ellison syndrome. MS is kind of strange idea. Gastrinoma should be ruled out at first. In case of gastrinoma, endoscopy should reveal peptic ulcers.
Nausea, acid reflux, difficulty swallowing can be caused by stomach or esophageal cancer. However neither of this diseases gives diarrhea. Is kind of hardly possible for the stomach cancer to give diarrhea. It may happen if fistula between stomach and large intestine will appear (but this is extremely rare).
Barretts esophagus is possible as this condition appears due to long time stomach acid irritation. This can be present apart from other conditions.
Gastroscopy is a very good idea and should be done as soon as possible. In case of peptic ulcers, also gastrine level should be checked.
Hope this will help. Feel free to ask further questions.
Regards.