... Place a reverse biased diode in parallel with the magnet coil. ... There will be very little switch arcing. Use a switch rated for the AC voltage and current. ...
It's usually a bad idea to use a switch that's not rated for DC on a DC circuit.
At the instant switch contacts open, and for several milliseconds more, the distance between them is small enough that the current can arc across the gap. This happens on all circuits, both AC & DC, and also non-inductive loads.
In a DC circuit, the current continues to flow until the gap is wide enough to extinguish the arc.
In an AC circuit, the current drops to zero twice per cycle and the arc goes out by itself.
Because of this, a switch can be used at a much higher AC voltage/current than DC. This is why switches (and fuses & breakers) often carry two different ratings -- 125 VAC/32 VDC, for example -- or are rated for AC only.
In this specific case -- rectified but unfiltered DC -- you might be able to get away with an AC-rated switch because unfiltered DC also drops to zero at the end of each half cycle, but an RC snubber might eliminate this advantage.
Since this switch will be turned on & off hundreds of times per day, I recommend using a switch or contactor rated for considerably more current than is actually flowing.
A snubber resistor can be used in series with a reverse-biased diode to speed up the turn-off time.
(the inherent resistance of the magnet coil might be an adequate amount of resistance)
The residual flux problem can be solved with a switch that reverses the polarity of the current through the coil, which the user can flip when it becomes a problem.