Eknath has a deep understanding of Indian spirituality, in first 75 or so pages he explains it beautifully. He also explains how Bhagwad Gita is a map of how to live a life. These teaching can be applied even today to live a good life without too much stress.
Yeah the uncoerced choice to eat chillis is a major potential confounding factor. It's really not for everyone and frequently seems to have something to say about the environment you were brought up in & the type of food you normally eat.
Yes I'd certainly add attitude and openness. Except I don't have any quantitative research to back that up, so you're just winging it at that point. Which ends us back exactly where we started.
GMA, work sample and integrity however DO have quantitative research to back them up.