Quantum pairing systems subjected to a sudden parameter quench can exhibit long-lived prethermal dynamical phases that are distinct from thermal equilibrium states. We investigate the post-quench nonequilibrium dynamics of a Bardeen-Cooper-Schrieffer (BCS) model with homogeneous pairing interactions using Keldysh nonequilibrium field theory. Starting from the closed-time-path functional integral, we introduce a complex auxiliary pairing field to decouple the four-fermion interaction and derive the effective action. Within the saddle-point approximation, the stationarity condition yields the self-consistent time-dependent gap equation. Using the Dyson equation in Keldysh–Nambu space, we further obtain a closed set of Bloch-type mean-field equations for the real-time evolution of momentum-mode occupations and anomalous pairing amplitudes. For a single-particle spectrum consisting of two subbands with homogeneous density of states, the dynamical phases are identified from the long-time average and standard deviation of the order-parameter magnitude over a prethermal time window after the transient regime. Three prethermal phases are found: phase I, in which the macroscopic order parameter decays to zero through dephasing among different energy modes; phase II, in which it approaches a nonzero nonthermal plateau; and phase III, characterized by persistent collective oscillations. The oscillatory phase is further divided according to the trajectory of the complex order parameter. In phase IIIa, the trajectory does not cross the origin and the order-parameter magnitude remains finite, whereas in phase IIIb it periodically passes through the origin, producing zero crossings accompanied by phase jumps close to \pi. The numerical phase boundaries agree with the analytical Lax-vector results in the Anderson-pseudospin representation. We also establish the correspondence between the theoretical pairing field and experimentally measurable cavity-output quadratures. The amplitude, phase, and spectrum of the output signal provide operational signatures for distinguishing the dynamical phases. At the mean-field level, the same self-consistent dynamical structure applies to weakly interacting two-component Fermi gases in the BCS regime, while quantitative applications to the BCS-BEC crossover and the unitary regime require the inclusion of fluctuations and collision effects. These results provide a unified framework connecting microscopic fermionic evolution, macroscopic dynamical phases, and experimental observables.