I just read Dr. Abram Hoffer's book on the Orthomolecular treatment of schizophrenia, and he liked hexaniacin inositol, which is what I take. He used niacinamide too, and found them equally effective.
Here is the difference between them: hexaniacin inositol will not produce a flush, whereas niacinamide will. Niacinamide also may cause nausea if one takes too much.
Don't know much about NADH, and it wasn't mentioned in the book. But the others work well, and are fairly cheap, so it makes sense to start there. It does take time for vitamins to have an effect on symptoms, though.
Hope it helps the young person!