Actually the theory of monads bypasses the mind/body dualism by asserting a kind of synchrony. Spinoza made the same move in a different way by claiming God is the multi-dimensional plane of existence whose varying realms (mind and extension [body] being just two of an infinite number) are completely synchronousness. You're also off with the Christian comment since Leibniz wasn't within the domain of the Catholic church.

St. Luke understood the brain to be the home of mental faculty and the situation never really changed. Especially once Aristotle re-appeared, the brain was definitely understood as an organ that processes sensory input can be affected like other organs. Caffeine, exercise, possibility of surgery all known. Brain speculated to be involved in comas. Existence of neurons speculated.