Question: If the Universal Wave Function definitely existed, would that mean the Many-Worlds Interpretation was automatically true or would it only imply that?

Answer: 
Measurement is happening all the time *only* in the macroscopic world. The fundamental difference between a quantum particle and a macroscopic body is that a quantum particle is not interacting with its environment, or if it is interacting then it is not interacting in such a way as to generate the measured property you are interested in.

The conceptual difficulty in quantum mechanics is that in the absence of measured properties, particles do not behave in a way which we find intuitive. Indeed, one must go into the mathematical foundations, not just study the results, if one wants to understand it. I have sought to clarify this in a published paper [The Hilbert space of conditional clauses](https://arxiv.org/abs/1205.4607).
I thought of an interpretation about this question and would like to know if it is meaningful:

Saying that there is no superposition of states is only true for a given orientation of coordinates. And the notion of superposition has only meaning for a given frame.

If a particle $A$ has spin up in a given direction it has a superposition of up and down for an axis not at that direction. It is not possible to choose a frame where all particles have no superposition of states.

It is simmilar to an elastic field inside a body in equilibrium. It is always possible for each point, to find an axis orientation such that there is no shear stresses in the stress tensor.

But if I choose that orientation, it is perfectly possible to have shear stresses at points in the neighborhood.