I cant advice much about niacinamide vs niacin. I take a low dosage of all vit B, almost daily. Tried the niacin flush early in the Iodine process, but had a hunch to quit.
About chemical sensitivity; I have had this always. Better with iodine, much better with sulfur (msm and others)- I can even use skin creams !! Seem to me like it (sulfur) opened the detox pathways.
Still sensitive for food additives , but hopefully this will not change; thats how the body alert us.