As I stated, because KE=1/2*m*v^2, it takes tremendous kinetic energy to reach orbit. A Mach 7 reusable booster is easier to design but it makes the orbiter much heavier. This is because the remaining delta-V is a squared term, hence energy required falls disproportionately on the orbiter.
No, you're missing basic physics. Acceleration is
directly proportional to force, and in this case force and thrust are the same thing (i.e. acceleration is force/mass) Work (or power) is force times velocity (in other words, the faster a rocket goes, the more work it does even if the thrust says constant). Energy is proportional to V squared, but acceleration is still directly proportional to thrust.
I won't bother with the rest of your analysis because it's based on a flawed assumption.