Why is there free magnesium in the ecliptic (if there is) and why would some be orbiting within the ecliptic plane but retrograde?
Mg I does indeed mean neutral magnesium atoms, but May and his collaborators weren't observing magnesium atoms orbiting in the inner Solar System -- they were seeing absorption by magnesium atoms in the Sun's atmosphere.

In the optical and near-infrared, the zodiacal light is sunlight scattered/reflected by interplanetary dust particles. Not surprisingly, the spectrum is largely the same as that seen coming directly from the Sun, complete with all the Fraunhofer absorption lines. Their approach was to focus on a narrow region of the solar spectrum containing a strong Mg I absorption line, and measure how the the dust particles redshifted or blueshifted the reflected solar spectrum, since that would tell them the velocities of the dust particles relative to the Earth.

(They also sometimes detected Mg emission partly filling in the absorption line; since this was always at the rest wavelength of that transition and thus not Doppler shifted, they deduced that it was probably emission from Mg atoms in the Earth's upper atmosphere, likely deposited by vaporized meteoroids.)

As for possible retrograde motions: the zodiacal dust is thought to come from three main sources: occasional collisions between asteroids, sublimation from comets passing through the inner Solar System, and a possible contribution from interstellar dust grains as the Sun moves relative to the local interstellar medium. The second source -- comets -- can give you retrograde orbits because many long-period comets (coming originally from the Oort Cloud) have retrograde orbits.