The reheat is needed to get past the transonic region and onward to supersonic. Once you've gone supersonic the drag actually drops a lot. So you want to get through the transonic region as quickly as possible and reheat is a good way to do it. After that the Concorde just supercruised.
I think so, many planes are like this: the transonic regime has the highest drag. It's not as simple as v^2, you have to consider the compressibility effects (wave drag where the pressure wave moves along with the plane, like a bow wave on a ship). Flying faster also means you can fly higher in lower density air, so the indicated airspeed (proportional to density * v^2) is reduced.
> It's not as simple as v^2, you have to consider the compressibility effects (wave drag where the pressure wave moves along with the plane, like a bow wave on a ship).
I think it is that simple. Those effects are the reason the drag coefficient is changing, you don't need to account for that twice.
