I'm still on Dr S's diet but have had a few relapses, which would count as reintroductions
I actually haven't noticed any adverse effects from these lapses, in general, and for this and other reasons I am a little sceptical that dietary factors are a major trigger (for me, anyway).
I'm not even convinced that fully caffeinated coffee is bad - mind you, I want to believe that because I really miss coffee and tea! Throughout the diet, I have had two decaffeinated coffees per day without apparent ill effect.
I think the obvious approach is to just reintroduce one food item at a time, and see if you notice any bad effects. But do it more than once and wait a few days each time, as apparently there can be a delay of a few days between trigger and effects.