is is the case with most health issues, certain deficiencies can cause certain issues. But, outside of having a specific deficiency, its all placebo.
Will taking vitamin D and B12, magnesium, etc. cure your AuDHD? No.
Could taking them make you feel better in some ways, freeing up some spoons/mental space to better handle whatever issues in your life that requires energy and focus to deal with? Yes, its possible. Assuming you are deficient, which, minor to moderate deficiencies in those three aren't uncommon.
Its not really cool to l lie about food allergies; doing so makes it less likely for people with food allergies to be taken seriously.
Though tbh, I avoid eating out unless I literally have too anyway, been poisoned too many times from I assume cross contamination or negligence.
(and, I am also vegan, which doesn't make 'eating out' easier)