You're welcome. I wouldn't get too hung up on finding the latest definitive research, as what we know today may change in three months, and even more this time next year.
Two of the papers I gave you are preprints which haven't yet been peer reviewed.
Respiratory viruses may not be new, but the COVID-19 virus is new. COVID-19 virus appears to spread more easily than influenza, which may change the equation.
From the Q&A section of the WHO website:
"As the COVID-19 outbreak continues to evolve, comparisons have been drawn to influenza. Both cause respiratory disease, yet there are important differences between the two viruses and how they spread. This has important implications for the public health measures that can be implemented to respond to each virus."
My understanding is that masks for the public are secondary protection, with social distancing being the primary recommendation. It is expected that masks have a limited, yet still beneficial effect, but it's more like frosting on the cake, rather than the cake itself. Research may eventually show how limited, but we are surely not there yet, and cannot wait.
Rather than battling with studies, we already have real world evidence of techniques that have worked to limit the spread of the virus, and also techniques that fail to get the spread under control.
Having said that here's another paper...
A paper and a video showing how different masks might provide better or worse protection, with the best mask limiting droplets from a simulated cough (I know -- not real) to 2.5" from the face, versus 12' unprotected:
https://aip.scitation.org/doi/10.1063/5.0016018https://www.livescience.com/face-mask-visualization-droplets-covid-19.htmlWith respect to your under-65 survey experiment, I live in a country where total cases and total deaths are quite low, and virtually no one questioned the recommendations from the government to stay at home, telework if possible, social distance, and wear a mask.
Even in America, with the high numbers, the cases are about 0.8% of the US population, and deaths are 0.04%, and under-65 deaths would be some small fraction of that. If I lived in the USA, I might need to survey 1000s of people to get some meaningful results.
But death is not the only metric -- some are falling sick, and they appear to have longer term, maybe permanent, damage, not making full recoveries. Imagine if you were a professional distance runner, who takes breathing for granted.