The third climb, cruise, descend would most likely only work for one given mileage. (say 240nm.) This is because the climb usually takes about 80 nm and so does the descent. On long haul of 3000nm it is unreasonable to climb or descend for 1000nm.
As for the speed question, there is something called 'Coffin Corner' which, at high altitudes limits how fast you can fly because of engine power, and how slow you can fly because of aerodynamic stall. At very high Flight Levels these two numbers get closer together.