To answer your other qestion, I's suggest you first read the FAQ on Potassium Carbonate, and then consider these additional points:
- to achieve the same ionic potassium contribution (136 ppm) one would have to add ~45% more bicarbonate (6.6g total). FWIW, the carbonate is ~3.5 times more soluble in water than is the bicarbonate form.
- Potassium bicarbonate has essentially the same stabilizing effect on pH as does the carbonate. Both dissolve readily in water, but if you add the bicarb to hot water, it will break down into potassium carbonate and... CO2.
Which of the two compounds would be better for mead fermentation? Although I've used both, I can't honestly say if one is better than the other...