multiplex numerous strips of WS2812
You're missing the fact that WS2812 do their own PWM'ing. Once you have loaded data into the strips, each individual WS2812 LED driver drives its LED with PWM, asynchronously to the other LEDs in that strip, and to the other strips. 

You could perhaps write 'on' data to one strip, then write 'off' data, and write 'on' to the next strip. But that seems like a hard way to get a flashing version of simply programming the LEDs to a lower brightness, which will also reduce the current draw of the entire setup.