You are doing fine. Your first job is to heal and learn about your new system. If you like oat meal and mashed potatoes, have some. Honestly you are not eating enough to make a difference. AFTER you heal and you can eat, you put the hammer down and lose weight. Then the only good carb is the one you didn't eat. Protein, the general rule is 30gr at 30 days, 60 at 60, and so on. It is more important that you stay hydrated. Drink all the time.
Those stupid vitamins Take what you have already bought and don't buy anymore. You need higher dose. There is a lot of information here on what you should get.
Diarrhea. Treat it. Imodium is your friend. Take some. I probably had Imodium with me all the time for a year of so. Didn't use it very often but it was there just in case.
Don't know about the hernia because I have never had a hiatal hernia. But I can imagine it would be hard to eat for a while postop.