Unlocking the full economic and environmental potential of lignocellulosic biorefineries hinges on overcoming the critical bottleneck of residual lignin valorization. This study introduces an integrated, solvent-free paradigm to directly transform enzymatic hydrolysis lignin (EHL) from corn stover into high-strength polylactic acid (PLA) biocomposites. By modulating the severity of an upstream green oxalic acid pretreatment (140, 180, and 220 °C), structure-property relationships dictating composite performance were elucidated. Comprehensive FTIR, GPC, TG, XPS, and the 13C CPMAS solid-state NMR analyses unveiled temperature-dependent evolution of EHL, with pronounced depolymerization at 180 °C yielded a highly mobile lignin matrix characterized by lower Tg (92 °C) and Tm (177 °C), whereas 220 °C triggered profound structural re-polymerization. Direct thermal blending of raw EHL with PLA, without prior chemical modification or solvents, increased the tensile strength. The optimal formulation containing 10 wt% EHL derived at 220 °C increased the tensile strength to above 60 MPa, while the stiffness remained comparable to that of neat PLA. Concurrently, the 180 °C pretreatment delivered an unparalleled operational balance between macroscopic property enhancement and holistic process sustainability. Under the stated model assumptions, the techno-economic analysis indicated potential economic competitiveness, with minimum selling prices of USD 2.65/kg (10 wt% loading) and USD 2.38/kg (20 wt% loading). Sensitivity analysis identified the PLA purchase price and EHL-PLA annual production rate as the principal economic drivers. This work demonstrates the potential of raw EHL as a biopolymer additive and provides a basis for integrating biochemical fractionation with thermoplastic engineering toward more sustainable plastics.