This work establishes a unified operational framework bridging Special Relativity (SR), General Relativity (GR), and Quantum Mechanics (QM) by introducing two fundamental extensions to relativistic mechanics while demonstrating explicit reduction to standard physics in correspondence limits. First, we resolve the historical kinematic asymmetry in relativity—where speed is universally bounded by while proper acceleration a remains unphysically unbounded—by deriving an invariant maximum proper acceleration scale directly from horizon dynamics and quantum localization limits. Second, we extend the Einstein Equivalence Principle into the quantum domain via the Unified Equivalence Principle (UEP), establishing a three-way operational equivalence. By modeling a massive particle as a localized quantum clock oscillating at its Compton frequency, the canonical Hamiltonian Legendre transform partitions total relativistic energy into an internal-clock component and a motion-dependent time-dilation component. We demonstrate that physical acceleration forces a progressive de-excitation of internal quantum clock states, where internal phase depletion acts as an equivalent acceleration manifesting macroscopically as observable quantum phase shifts. Applying the Heisenberg energy—time uncertainty principle across the co-moving Rindler horizon demonstrates that the maximum acceleration amax corresponds to the complete evacuation of the clock to its vacuum ground state. At this critical threshold, the Rindler horizon stabilizes precisely at the reduced Compton wavelength, preventing geometric collapse into naked singularities. In the standard physical regime, the framework reduces continuously to standard Special Relativity, General Relativistic weak-field potentials, and Newtonian kinetic dynamics.