M1: The atom has a wavefunction, a function from {up,down} x R^3 to C. We can view this as two spacial wavefunctions (i.e. from R^3 to C) one for spin up, one for spin down. Before entering the field the up and down wave functions are the same, and localised in space (i.e. all the amplitude is in a small lump around a point). This lump of amplitude moves along until it reaches the field. At this point the two wavefunctions cease to be the same (the spin of the particle becomes entangled with its position). The one associated with spin up translates upward, its velocity increasing just like a particle in a classical field. Similarly the spin down wavefunction moves downward. The time that the worlds take to split is the time until the two lumps (corresponding to up and down) cease to overlap spatially. I don't view the "worlds" as ontologically fundamental, only the wavefunction, so the previous sentence is close to tautology. If we allow the wavefunction to have thin tails off to infinity then the lumps never truly split, but they do still mostly separate.

Since the two lumps haven't separated very far (say at most a few cms, or however big the S-G apparatus is) and the atom hasn't entangled with anything else, it will be easy to remove the entanglement between spin and position by reversing the field. This is what I mean by "the worlds aren't very far apart". To formalise the notion of distance I suppose one could take the root of the sum of the squares of the displacements of every particle in the universe between the two worlds. So in this case the distance between worlds would be the literal distance between the two lumps of amplitude.

Once they hit the screen we suddenly have that lots of particle's positions differ between the two worlds, and so the distance between them becomes very great. This is decoherence.

M2: In the experiment given, any single particle is essentially returned to exactly the same superposition of states it was in before it entered the fields. Exactly like neither of the fields was ever there. (Also, I hold that I can still use density matrices to deal with my subjective uncertainty, even about single particles).