If there are many actual physical worlds out there representing all possible states of the wave function simultaneously, then the wave function couldn’t collapse because then those worlds wouldn’t exist. Each possible state of the wave function is a physical world representing that state.
Essentially, yes. I think the important point is that MWI is only concerned with the multiverse that an uncollapsed wave function represents, not any other kind of multiverse that might exist in science or philosophy.
Could you link the experiments which have definitively shown objective collapse and not just an entanglement illusion? Fair warning, I may need to ask for a layman’s explanation of how they proved the collapse was objective and not just the aforementioned illusion.
Here's a reasonably good article about them.. But to try and give a short explanation, the experiments were for a class of objective collapse theories were individual particles collapse spontaneously with a certain probability, and take any particles they're entangled with with them. The probability of any one particle collapsing at any given time is extremely low, but a macroscopic collection will collapse almost instantly, in the same way a uranium atom will take millions of years to decay on average, but a chunk of uranium sitting on a table will make your gieger counter sound like it's full of bees.
The important part though, is that - for reasons that are quite technical - the collapse of the particle actually emits a small but measurable amount of radiation, which is what the experiments were looking for.
To be clear, they didn't find it, which is bad for these theories. But if they had found it, it would have falsified Many Worlds.