I don't think you're right about the many worlds interpretation here. In the most popular "simple" many worlds interpretation, the whole universe is a QM syste

I don't think you're right about the many worlds interpretation here. In the most popular "simple" many worlds interpretation, the whole universe is a QM system described by the universal wavefunction. The "worlds" are just "branches" of the wavefunction that are not entangled with one another. The Schrodinger equation (and the more advanced QFT versions) are fully deterministic differential equations: the state of the universe at time T is fully determined by the state of the universe at time T-x, for any x. This remains true even if x is negative: there is no "arrow of time" in QM, there is no distinction between the past and the future in any QM equations. The only "non-determinism" in Many Worlds is thus of the knowledge kind: you don't know which "world" you happen to be in, so you don't know which result you'll notice. But this is like being spun around a coin: you don't know which face you'll end up looking at, but you know both faces actually exist. All the possible outcomes of any QM event are realized in "some world", there is 0 uncertainty.