Hi
Q1) The statement if not true, to begin with.
But I understand it as "Why do jets fly high?". The reason you mention is basically true but not fully explained. Thrust matches drag. At high altitudes you have more TAS for a given EAS. Drag is proportional to EAS squared. But specific range is directly proportional to airplane's speed, so at high altitude we have good value for money (better speed for a given drag than at low altitude).
To sum up, at high altitudes we have a better specific range (more miles per ton of burnt fuel).
However, MN also influences drag, so the whole thing is more complicated, and in a set of conditions there is an optimum level. Flying higher or lower than that means flying less efficiently.
Q2) I have no idea if it is right or wrong
Q4) the convergence-divergence theory is not applicable everywhere and it is not true that there is an upper high above a lower low, or the opposite. Wind behaves in a very different manner depending on latitude and other variables.