No, use the entire packet for a single dose of SO2. The formulation (therefore dosage), can be affected if the granules absorb any moisture.
No, do not break the tablets for smaller dose additions. The combination of potassium metabisulfite and potassium bicarbonate may not be evenly distributed in the tablet. The tablets are available in two sizes to help give dosing choices.
You have 22 ppm total S02 added. The amount of free depends on pH, residual sugar, solids, etc.