LDN is Low Dose Naltrexone and there are numerous websites devoted to it. They can tell you way more than I can. The one from Briton is very good. They are way ahead of us when it comes to LDN.
The very low fiber diets are for you are in a flair. There maybe only a few foods you will tolerate. Start very slow and find out. I use instant mash potatos (milk is a question if you are intolerant), jello, sometimes with yogurt mixed in if you tolerate it. Remember to let the jello cool a bit so you get the active part of the yogurt. I'm not sure what rich foods are so I don't worry abut that. Easy to digest is what I am after and calories, we need those. After a few days and things have calmed down, you can add foods from the low fiber list. As to nutrition, I have a lot of vitamins I take, but not every day. You will become an expert in what you need and where to get it. Read, read, read!