on a scale that’s grand enough to make the proverbial apple fall upward

Not necessarily. An apple teetering on the edge of a cliff requires no grand change in initial conditions to have two very different journeys. If “you” are a metaphysical entity capable of altering the signals in your physical brain, your brain could deterministically amplify and enact your will, like gravity does to the apple on the cliff. If you have a metaphysical existence, this is a pretty reasonable mechanism for it to work.