There are times when I do feel like a short order cook. This is usually when one of the children is sick (we have 3). When they are sick, I may cook them what they want to eat just to get something in them.
The rest of the time, I choose the menu (or my husband does). We try to make at least one side dish each child likes. My middle child (firstborn son) likes peas for example but my daughter hates them. So I may make an extra vegetable when I make peas for my son. But my husband's policy is that you need to eat what is put in front of you. There is nothing that we make that they don't like in some form for the most part. He has instituted that you need to taste any new food we make and you will probably need to eat half of what is put on your plate.
Along with the food issues, we had whining over the food. Since my daughter whined about the one for over an hour while procrastinating eating it, she was told that if she did not eat it for dinner, she would have to have it for breakfast. My husband ended up covering it and feeding it to her for breakfast the next day. She sometimes whines now but not nearly as much or as often.
But honestly, there are days when they don't eat very much. That happens with any child. They grow in spurts and sometimes need more food and sometimes less. If they are healthy for the most part (I know you have some other issues as well) and the doctor tells you they are growing fine, then I wouldn't worry about how much they eat (except for the one with Prader Willi). We cook a balanced meal with protein (meat, fish or cheese), starch and vegetables (and sometimes fruit). Like I said, we try to make sure we have at least one thing at each meal that each child likes. This way we know that they will eat something during the meal.
Also, meals are a time to sit down and not do anything else but eat our food. When they are done (to our satisfaction), they are supposed to scrape anything left on their plates into the garbage and put the plate into the sink. They are supposed to check the table to make sure there is no food on it as well. We are still working on this (ours are 8, 3, and 9 months), though. Dinner never goes perfectly but we do pretty well and they usually eat pretty healthily.
I know you are dealing with a lot of issues. Just try to be patient. You may want to plan your meals for the week ahead of time. But behaviorally, take it one meal at a time and it will get better with time.
Good luck. Hope this helps.