You captured it well. I just think concussions and head impact are less unique to football than the combination of the three factors and that researching the interaction of the three would probably be very helpful to the people who are suffering degraded health. My worry is that answer does not lead to easy headlines or simple stories that can be captured in a one hour piece of programming, so there is little motivation to put forth the huge cost of researching all aspects of the problem, and worse (from the NFL/owner's perspective), not having players use PEDs or narcotic painkillers.
Let's nip this in the bud before we get crazy here. Head trauma is factor in dementia much in the same way that cigarette smoking is a factor in lung cancer. There are certainly other factors that are important (e.g. genetic factors), but lets not just grab on to anything that NFL players do as a sign that it leads to dementia.
CTE, like all dementias, involves a cascade of cellular (e.g. soluble misfolded proteins trigger a cascade of effects that leads to cellular excitotoxic(literally brain frying) death) and system reactions (e.g. the spread of soluble misfolded proteins occurs in areas of the brain that co-activate in different task states) that end in neural atrophy. The systems of the brain that are affected vary from dementia to dementia and within dementias. As far as I know, none of these effects have much to do with opiod receptors, or opiod ligands. If anything, the anti-inflammatory effects of analgesics may slightly reduce the risk of dementia, or have very little effect.
Furthermore, saying that you want to examine something holistically is not as easy as actually doing it quantiatively. Opiods present a confound, in that people with dementia are more likely to be taking opiods due to indirect effects of having dementia (e.g. physical injury or other aging problems). In order to determine whether opiods present a risk factor, you need to control for the others.
Finally, a recent prospective study that did just that in 3000 individuals found little evidence that opiod use is a risk factor for dementia, which is consistent with what we know about dementia in the first place.
On the other hand, there's strong evidence to suggest that, yes, head trauma is a huge factor in subsequent neurological/psychiatric disorders. It is established as a large risk factor in most dementias. In fact, mouse models of head trauma mimic the development of pathology observed in most dementias (e.g. tangles, specifically neurofibrillary tangles of phosphorylated tau).
Other professional sports likely involve heavy usage of PEDs (whatever that means -- some PEDs may have neurodegenerative effects, and they are NOT the ones marciano uses) and opiods. So far, we've seen no indication that dementia is higher in those other sports. On the other hand, boxers suffer from dementia that is extremely similar to CTE (previously known as dementia pugilistica for that very reason), which again implicates head trauma as one of the largest environmental factors.
EDIT: I apologize for the glibness of my post. If I wasn't in the middle of moving to Portland, I would post links, etc. Sorry about that.