The macroscopic piezoelectric performance of ferroelectric thin films, particularly in Micro-Electromechanical Systems (MEMS), is significantly governed by extrinsic contributions from ferroelastic domain wall motion (i.e., a/c domain switching in the tetragonal phase). However, quantitatively tracking this domain evolution under the unique mechanical boundary condition of substrate clamping and correlating it with macroscopic electromechanical responses remains a critical challenge. In this work, we present a cross-validated methodology to quantitatively deduce the dynamic volumetric fraction of c-domains in epitaxial Pb(Zr_{0.3}Ti_{0.7})O_3 (PZT) thin films. This is achieved by combining structural evaluation via X-ray Diffraction (XRD) peak area deconvolution with functional evaluation utilizing an internal-bias-corrected symmetrization protocol for Polarization-Electric field (P-E) hysteresis loops. To physically interpret these precise experimental observations, we employ an existing multi-domain electromechanical constitutive framework and related domain evolution concepts. By integrating Landau energy, mechanical elastic energy, and electrostrictive coupling, this theoretical framework is utilized to effectively simulate the energy-driven polarization switching and domain activities under substrate constraints. Our combined experimental and computational approach elucidates the distinct competitive mechanisms between 180° ferroelectric domain migration and 90° ferroelastic domain migration under cyclic electric fields. The constitutive analysis clarifies that substrate clamping induces significant localized mechanical elastic energy storage during a/c domain switching. Upon the removal of the external electric field, this stored mechanical energy acts as the primary thermodynamic driving force for domain "back-switching," dictating the reversible macroscopic response. This study demonstrates how initial in-plane domain fractions and substrate-induced elastic energy dictate piezoelectric performance, offering physically-grounded insights for designing advanced piezoelectric devices and superlattices.