Switching the AC mains is done at the line frequency, especially when using TRIACs. Using PWM to switch DC can be done at 10-20KHz easily, much easier to filter. Depending on the current, the PWM is unlikely to get back on the mains with a sufficiently sized capacitor on the DC bus. There will be some harmonic distortion on the mains when using just a plain diode bridge rectifier, again depending on the current. This is how most servo drives and inverters work, although this application should be much simpler as it is driving a resistive load with no back EMF to deal with. There would certainly be no “flicker” and the system could be designed with as much resolution as required.
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