kmeta (potassium metabisulfite, in your case condensed form camden tablets) is used to suppress yeast and bacteria that you don't want, and also provides protection against oxidation. The cultured yeast you buy is much more tolerant of the sulfite, so it can handle larger amounts. You can definitely stun a fermentation with too much kmeta, but it is not used to control the fermentation, but only used to protect your wine from unintended organisms and O2. Potassium sorbate is what is used to prevent re-fermentation by not enabling the yeast to reproduce. So as the existing yeast dies out, the fermentation slows and stops completely because there are no more yeast cells to replace them. Sparkling wine is just keeping the wine contained and allowing the CO2 generated by the yeast to build and eventually saturate it with bubbles. So proper levels of sulfite will allow for fermentation, and thus will allow you to make sparkling wine.