If the Universal Wave Function definitely existed, would that mean the Many-Worlds Interpretation was automatically true or would it only imply that?
It would imply that we are entities bound by the laws of quantum mechanics, and thus there are no truly classical observers to observe the system. The QM interpretations are only needed to explain their behavior with respect to a classical observer.
Decoherence based theories would be the natural winner.
The concept of "wavefunction"is inherent in the definition of Quantum Mechanics,in the same way the concept of mass is inherent in the definition of classical mechanics: it is a mathematical function which is used to model data and predict physical behavior of matter at small dimensions .
Present physics modeling assumes that the underlying microscopic interactions obey quantum mechanical equations, and it has been shown that the classical theories emerge from this underlying level. In this sense a universal wave function, assuming extreme measurement accuracies, exists and has been modeled with the density matrix formalism .
[The many-worlds interpretation] (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wavefunction collapse. This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe
As in principle a universal wave function exists, the crucial word is "interpretation". It means that the proposal gives exactly the same numerical predictions for new studies , just a different way of interpreting the internal functions.
would that mean the Many-Worlds Interpretation was automatically true or would it only imply that?
The universal wavefunction exists, can be formulated mathematically . The mathematics is true, but the many-worlds interpretation of the mathematics is just a different optical angle,there are no new predictions to be able to distinguish if it is valid or not.