I think the bulk of this is made up of questions for the doctors. It really sounds like you are trying to diagnose a little too much by counting heartbeats and working with water, magnesium, alcohol and so on, and suggesting Celiac vs. IBS.
Stop with the magnesium supplement - for both of you. There is no basis for giving a single nutrient (vitamin, mineral, trace element, whatever). You can make things worse by doing this. First of all, you don't know where that supplement is made (you can tell from the label who distributes it, but not where it's manufactured). Secondly, nutrients don't work in a vacuum. There is a complex interaction among essential nutrients and they don't work or a person can overdose if these items are taken singly. Even overdoing the water can be a problem - yes dehydration is a problem for many people, but drinking too much can cause health problems too. (Remember the recent death from someone trying to win a radio contest by drinking huge quantities of water?) In addition, any supplement (or prescription) in pill form is of very limited use - absorption levels are very low. There's not much you can do about prescriptions, but there is a lot you can do about those over-the-counter minerals that primarily make huge profits for the companies by convincing people who are "low" in something to just take more and more of it. That's not the way cellular nutrition works, and that's not the way the body absorbs nutrients. So they are just eliminated, which is a waste of your money, or they are stored without being eliminated, which is totally unhealthy.
There is a huge body of research and food science, complete with clinical trials, that supports what I'm telling you. Comprehensive supplementation should be done responsibly and by looking at things like patents (very rare on foods) which prove safety, effectiveness and uniqueness. There is a patented supplement that deals with digestive issues such as IBS - it can be taken by some people with Celiac disease as well. There is another comprehensive supplement that often takes care of these issues even if Celiac disease is present. If you want to explore this, I'm happy to advise you.
Improving his eating is a good idea and I think some nutritional imbalances are probably linked to his situation, as well as to your anxiety. And of course his heart issues are probably increasing your anxiety, which is why you are being hyper vigilant regarding listening to his heart. Making smoothies and general improving nutrition is, of course, always a good idea, but it's almost impossible to get the full range of essential nutrients even that way (which is what the AMA said in 2002).
Happy to give you more info along these lines if you are interested, but in the short run, please don't continue!