I guess for a low carb diet, I'd rather see oatmeal
than potatoes... at least, oatmeal
is a whole grain, so you're getting fiber with it and it takes longer to break down, so it's not going to spike your blood sugar. Mashed potatoes, on the other hand, will spike the blood sugar and increase the demand for insulin very quickly.
No, the tests your doctor did won't tell if you have H. Pylori or any other parasite. Those have to be tested specifically, by name.