Welcome, Phlox. Glad you've found us. Just remember that we are not doctors, just patients, and have all had different experiences.
Some people have intolerance to sulfites in lotions and beauty products (again - NOT the same as sulfate). This can come out as a rash. Beware of sulfites in medicines also.
First thing you need to do is figure out your tolerance - as long as you don't have an actual allergy. The website above gives you directions on how to calculate how much sulfite is in a portion of food or drink. I have figured I tolerate about 2000 micrograms a day. When you know how much is in different foods, you can tell how much you can eat. This goes a long way to lifting the limits on your diet! I can have an occasional soda or a slice of pizza (depending on the toppings).
Digesting carbs is probably not related, although I don't digest them well either. I take a cheap generic pancrelipase capsule with each meal that has fat or white carbs - especially bread. Speaking of bread: you could also have an intolerance to wheat. I have figured out it bothers me. There's nothing preventing us from having more than one condition! I do fine with rice, oatmeal, fresh potatoes, other starches. French fries are sulfited to keep them white here.
I don't have asthma, but a lot of sulfite intolerant people do. I do get heart arrhythmia and tachycardia from consuming sulfite beyond my tolerance. Look up cardioesophageal reflex. Heart and stomach, esophagus are connected by vagus nerve. It explains a lot about IBS. I was in the ER once when I had an abscess in my colon which caused a bad tachycardia. I had the heart converted with adenosine, but I ended up in hospital 10 days on antibiotics for the infection.
Water: It shouldn't be a problem if it has sulfate, but some water could indeed have sulfite if it has a little sulfur dioxide from a coal seam or sulfur spring.