We present a comparative optomechanical analysis of a tunable, monolithic Littrow wavelength-division multiplexer (WDM) integrating silicon nitride waveguides with an enhanced concave diffraction grating. Three device configurations, distinguished by their mechanical boundary conditions and actuation locations, are investigated using coupled finite-element mechanical modeling and fullwave electromagnetic simulations. The analysis establishes quantitative relationships between applied force, structural deformation, and spectral tuning across the extended C-band. Results show that structural stiffness strongly governs the tuning behavior. The fixed–fixed configuration provides highly linear wavelength tuning, stable channel spacing, and minimal mechanical displacement but requires significantly higher actuation forces. In contrast, the fixed–free and fixed–anchored–free configurations reduce structural stiffness, nearly doubling the wavelength sensitivity to the applied force, though with mild nonlinearity and increased displacement. These configurations also partially restore near-linear spectral behavior. Across all configurations, a clear trade-off emerges between mechanical compliance, tuning sensitivity, and structural stability. Increasing compliance reduces the required actuation force and improves tuning sensitivity but requires larger actuator travel and may affect mechanical reliability. These findings provide quantitative design guidelines for optimizing mechanical boundary conditions and actuator strategies in compact, energy-efficient, and low-loss monolithic microelectromechanical systems–photonic WDM systems. Received: 12 March 2026 | Revised: 14 May 2026 | Accepted: 17 July 2026 Conflicts of Interest The authors declare that they have no conflicts of interest to this work. Data Availability Statement Data are available from the corresponding author upon reasonable request. Author Contribution Statement JiaMehdi Kharazmi: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing – original draft, Visualization. Pierre Pottier: Software, Validation, Investigation, Resources, Data curation. Jayan Ozhikandathil: Software, Validation, Investigation, Data curation, Writing – review & editing, Supervision. Muthukumaran Packirisamy: Conceptualization, Software, Validation, Investigation, Resources, Data curation, Writing – review & editing, Supervision, Project administration, Funding acquisition.