Q1. The tangent to the curve gives the lowest ratio of drag to TAS. Try taking figures off a graph and working out the ratio. Further up the curve will give a higher TAS, but at the expense of a disportionately higher drag. Further down the curve will give lower drag but with a greater loss of TAS.
Q2. There is, I believe, no mathematical reason why it should be 1.32. Since modern jets are pretty much clones of each other I suspect that 1.32 is a working figure which will apply to most aircraft