1. Potassium reacts with water preferentially to produce hydrogen in this reaction as it is very reactive, then replacement reaction would occur between sodium ion and potassium, hence
2K + 2H2O = 2K+ + 2OH- + H2
K + Na+ = K+ + Na
Summing up the above equations would form hydrogen gas and KOH
2. Zinc corrodes in air easily but lead only corrodes in air in heat. Zinc doesn't sacrifice its electrons to prevent lead from corrosion in this case.
3. Ammonium chloride is covalent compound with a dative bond, so it doesn't ionise and it's not an electrolyte.