for Q1:
the partial pressure of any gas is the product of the mole fraction and the total pressure.... Initially, when t = 0, the only gas present will be A and thus the mole fraction = 1 and the pressure exerted by the gas molecules of A = 1300 kPa. As the reaction progresses, A decomposes to form B and C, and thus the mole fraction decreases from 1 to lets say 0.8 and so on.... and thus the partial pressure decreases as the concentration of A decreases.
Because the above is true, the concentration of A is proportional to the partial pressure of A and thus you can plot a graph of partial pressure against time and it will be an indirect method of plotting concentration against time.
For Q2:
Use HCOOH in excess and then its concentration will remain constant.