How old is your "intense" kid now? I believe my stepdaughter falls in that category, and my wife and I seem to have the same worries you do (or did) about ADD and other psychological ailments.
She's clearly very smart. She's really good at math (she's in first grade but already has a good grasp of 3rd and even some 4th grade stuff). It took a while but now she's good at reading. She's creative, usually kind to others, VERY social (a challenge for both of us being introverted and shy to an extreme), and doesn't cause trouble anywhere.
However, every conversation with her is a challenge. She's stubborn, sticks to absurd ideas and gets really angry if we try to show her she's wrong. This is contradictory with her ease at math, but she has a hard time following logical things like if A -> B and B -> C, then A -> C, so there are daily verbal arguments with her that are very frustrating. It's practically impossible to have a 5 minute conversation with her without it drifting into nonsense.
I would say the main difference between my boys at each perspective age was the speed with which they could process reasoned input. The chill guy by age 2-2.5 or so would listen and generally abide by instructions almost immediately to the best of his age ability, the intense one would take days or weeks at the same age to get to the same level of practice and disagree and try to argue in favor of his ideas or actions. It was very frustrating at first (I would have to take a walk to cool down) but then I would catch him a few days later behaving in the directed way. I didn't say anything about it, but I was just wtf?? After all that, now you're fine? Slowly I saw the pattern emerge that he WAS processing the information, but that the appropriate behavior came later. Sometimes weeks. That's when I stopped worrying about it so much. He 'gets' it, he just needs time to process it in his own way. I'm not sure I handled it the best, but I would calmly explain the desired behavior or information and leave it at that. I acknowledged that he disagrees and move on. Honestly, I worry more about my integrity as an authority figure than his displeasure or the rightness or wrongness of the issue being discussed. (That's what I meant by the long game) I do a lot of image processing in my side project and his behavior reminds me of image thresholding. He needs to see or be exposed to an ideas a certain number of times before it takes. I have no authority to offer any real advice, other than I understand your frustration.
She's clearly very smart. She's really good at math (she's in first grade but already has a good grasp of 3rd and even some 4th grade stuff). It took a while but now she's good at reading. She's creative, usually kind to others, VERY social (a challenge for both of us being introverted and shy to an extreme), and doesn't cause trouble anywhere.
However, every conversation with her is a challenge. She's stubborn, sticks to absurd ideas and gets really angry if we try to show her she's wrong. This is contradictory with her ease at math, but she has a hard time following logical things like if A -> B and B -> C, then A -> C, so there are daily verbal arguments with her that are very frustrating. It's practically impossible to have a 5 minute conversation with her without it drifting into nonsense.
Do you notice the same in your "intense"?