And seeing how families go through grief, I think determining the cause of death would be the last thing on their minds.

That's the point. No study will be possible because of their reluctance - it's understandable yet sad - that is not a criticism, merely an observation. In the interests of advancing knowledge, if it happened to one of my own loved ones, I'd let investigators (medical) do their thing as long as it didn't violate the last wishes of the decedent. I'd be inclined to want to try to help prevent it from happening to someone else. There must be SOME medical reason that bloke died while most people do survive. Maybe it was just senescence. 

I also hear that consuming foods containing gluten can contribute to dementia, although research seems limited. I don't know if this is true.

 My research in this regard is that there is no strong (compelling) medical evidence of harmful effects of gluten EXCEPT in people with Celiac disease. In which case one would have been told this by doctors early in life - it is not something that one "develops". It's a genetic disease with obvious symptoms. as far as I know it's very rare. 

It seems I hear scary things about the foods we eat everyday.

Some of it is paranoia, some justified. I have made a lifelong study of nutrition and such things. I do not guess. I research and base opinion on science, both empirical and theoretical. One of my reference texts goes back to the 1930s (Jethro Kloss, et al) based on years of successful folk medicine. The proteins we consume are used to build tissue. Amino acid ratios vary according to type of protein source. Meat is one of the only COMPLETE protein sources - it contains all the (known) amino acids required to build healthy tissue. One must combine non-meat protein sources properly to obtain complete protein availability in one's diet - but maybe there are others that ONLY come from meat which have to do with immune-system strength.