There's a couple of points I'd like to raise about your working.
According to the article the CFLs that they tested have a power factor of 0.45-0.50. An incandescent light bulb, being purely resistive, has a power factor of 1. Running at the same voltage twice the current will be required and this will manifest itself in I^2/R loses in the transmission lines and VA being required from the coal plant.
The internet tells me that a 23W CFL is about equivalent to a 100W incandescent. The CFL will still need to be powered by something, so powering for the 20,000 hour lifetime you assumed before, the coal plant will still produce ~35mg of mercury from said coal. So ignoring the impact of the reactive load of the CFL the amount of mercury used is 39mg.
However due to the poor power factor the CFLs will require reactive power, hence more current. I don't know the relationship between grams of coal burned and the power factor, but one can assume that the mercury that is produced will be greater.