This the speculative hypertemporal hypothesis, that posits an alternative temporal direction, that would somehow be angular to past-present-future of the entropic block universe. Nothing of the sort has ever been observed or experimentally suggested or demonstrated. It's a purely philosophical contention at this point in time.

Everett's argument was simply that the wave function is a real feature of the natural world, and that alternative states of a given amplitude ought also exist somewhere. There are, in turn, multiple interpretations of this interpretation.

It might also be that alternative amplitudes exist in other worlds of some inflatory multiverse beyond the cosmic horizon - in which case, the evolution of the wavefunction remains purely deterministic within our own block universe. Thus, free will and human decision-making need not apply. Everett himself was determinist in his thinking, and the beauty of his interpretation is that it's entirely consistent with observation.

Incorrect. The Many Worlds interpretation directly posits that collapse never actually happens; instead all wave functions are equally likely to occur, somewhere. This is the core pillar of the entire concept - that the wave function is real and that we should trust what the logical evolution thereof tells us.

The Everettian argument is that wavefunction collapse is a concept of human arbitration, and that it has never been observed in nature. In fact, experiments have managed to put ever larger systems into states of superposition. Instead of collapse, modern Everettians suggests the observed becomes entangled with the observer, which in turn results in the given amplitude, but alternative states of observation also exist - perhaps in some speculative multiverse of inflatory cosmology.