Zazen is very much a physical practice: the body is never an insignificant detail, as if meditation were a matter of mind and spirit apart from body. Why do we walk so slowly during kinhin? So slow that I often feel I will lose my balance? The point is to pay close attention to body, breath, and mind when you are walking just as when you are sitting.
Can you tell when a person is “more spiritually developed”? Does it show? I guess I have just defined an enlightened person as someone with wisdom and a good heart. Wisdom in Zen means the capacity to see that “form is emptiness, emptiness is form,” as the Heart Sutra teaches. What would this “wisdom ad good heart” look like? Probably like the spiritual qualities that all our great traditions have always prized: humility, kindness love, patience, forgiveness, understanding.
The important thing about the teaching of rebirth, the part that seems true and that matters a great deal is that life continues. That is, there is more to our lives than the little span of time between birth and death. The teaching of rebirth tells us that our life and death are significant beyond their appearances, more significant than we know.