> They don’t actually collapse, which is what the many worlds theory essentially posits.

That's not consistent with classical physics, and it is not what MWI actually posits. The universe branching that MWI posits is not really different from wave function collapse, and decoherence also doesn't really solve things. You still fundamentally have a single position for any classical system, but multiple positions with different amplitudes for quantum systems, and some mysterious threshold where you pass from one to the other.

Probability also can't really explain this, since the classical model also applies to single particles after they have interacted with a macroscopic system.

MWI considers that observers inside one branch can't interact or notice observers inside other branches, and this is why we perceive the world as if objects have unique definite positions. But this still doesn't hold up for quantum systems, which do in fact perceive and can interact with all of the other "worlds", including interacting with themselves in other worlds such as in the single particle double-slit experiment. So MWI doesn't really get away from the duality in any rigorous way.

By not collapse I mean no specific outcome is selected. The waveform simply gets directly translated as the distribution of universes. With effectively infinite universes the odds of someone being in an unusual one are directly proportional to how unusual it is.

You can’t experimentally differentiate between MWI and single universe wave function collapse.