We don't know anything about those children's brain activity, so this doesn't affect the argument. It's just irrelevant.

Even if the hyperactive adults varied in the severity of their symptoms, they all generally had lower levels of brain activity in the premotor cortex. This doesn't give us any reason to doubt the hypothesis.

This doesn’t change how much we can trust the study, so it's irrelevant. There’s no reason to believe those same neuropsychologists had to be present for the experiment to be performed correctly.

This gives us an alternative causal explanation for the correlation the researchers identified. Premotor cortex activity is diminished by a treatment for hyperactivity, which casts doubt on the hypothesis that diminished activity causes hyperactivity.

It doesn’t matter what’s harmful to the participants, or why the researchers only tested adults. This isn't relevant to the causal relationship between hyperactivity and brain function in the premotor cortex.