The symptoms you describe go beyond a vivid imagination, and are typical of children who display any of a number of anxiety disorders (e.g., generalized anxiety disorder, separation anxiety disorder). The signs are those of a biologically -based emotional disorder, and medication will actually probably be useful (even in addition to the ADHD, and even in that regard there are options). It would be wise to continue the pursuit of professional intervention. On the nutrition front, there may be dietary adjustment that would prove useful, though probably not decisive. In general, reducing carbohydrate intake and increasing protein intake, along with provision of omega-3 fatty acids can ofen be helpful. Yes, for a child with ADHD and/or anxiety, eliminating caffeine can be useful. The matter of refined sugar is controversial. Overall, it hasn't been demonstrated to be a primary factor in children with attentional problems, but on the short-term can certainly excite some children and diminish their capacity for self-control. And,the fact is that too many children ingest too much sugar.