I think a major source of confusion may be the fact that lift and drag are mathematical constructs: there to make the concepts/sums easier. The "reaction force" vector equals the vector combination of gravity and thrust in steady (unaccelerated) flight; any imbalance between these forces will result in a velocity change.
In the lift/drag model, energy is expended by moving the "drag force" along the flightpath; this is lost to the aircraft as a system and expresses itself in heat and motion of the air. Energy is also traded w.r.t. gravity, but as this is a conservative field, returning to the same point will leave no net expenditure.