Flaxseed is only one dietary item out of many and no one food by itself is going to do the job of preventing heart disease. You need to look at your overall diet.
It doesn't matter if flaxseed oil does everything that they say it does; the point is the overall health decisions that you are likely to make that will all combine for one great effect.
Flax as seed and oil is just a food. It alone will not cure anything, especially if the rest of the diet is full of processed food-like substances which hinder the body's ability to keep itself healthy. If taking flax as seed or oil starts you on the path of improving your diet overall, then it's a good thing.
I put ground flaxseed in my morning breakfast, and incorporate it in my homemade gluten-free bread. It gives protein, good fats, fiber, and probably lots of vitamins and minerals. I don't expect it to cure anything. I need to maintain a good diet so that my body has the best chance of fighting against health issues.
I have ADD (one of the conditions on your list). I lack certain neurotransmitters in my brain. Flax doesn't provide those missing neurotransmitters. I have a prescription for that. But flax is a part
of my overall healthy diet, and I know that when I get too far from the healthy stuff, the ADD symptoms are worse.
I am glad that you are researching the evidence for certain foods (thank you for reporting on your research efforts!), and I'm glad that scientists are attempting to look at them. I just don't think that one specific food is going to cure conditions that may have been caused by an overall unbalanced diet, or that were caused by genetics (ADD in my case).
Thank you for an interesting topic!