In the many worlds interpretation, as soon as a world has "split"/"the wave of differentiation has hit"/"particles have been entangled"/"whatever terminology yo

In the many worlds interpretation, as soon as a world has "split"/"the wave of differentiation has hit"/"particles have been entangled"/"whatever terminology you want to use" from the observer, it has zero impact on any future results, not just a small impact. Until that split happens, we're just talking about our own world. It's never possible to observe any evidence for more than one world, that follows from the way we define what a "world" is. Many-worlds does not try to explain why particles have a wave function, it only tries to explain the source of the apparent randomness in the way that wave function collapses, whilst removing the need for experimenters to be in some way "special" and immune from quantum effects.