This study investigates the dynamic behavior of the stapes stimulated by a round-window stimulating active middle ear implant. Initially, a linear mechanical model of the implant coupled with the middle ear was validated using cadaver head experiments to establish baseline accuracy and ensure it reflects physiological conditions. Following validation, the linear stapes motion under implant stimulation was examined, offering insights into the influence of the implant's design parameters on the stapes' dynamic response. To address the mismatch between the round-window niche length and the actuator length, shape memory alloys were incorporated to develop a nonlinear mechanical model. While shape memory alloys enhance adaptability and accommodate patientspecific variations, they may also introduce nonlinear stiffness, which could lead to instability in system motion. To address this, the system behavior was analyzed across implant design parameters such as excitation voltage and coupling rod stiffness. The results indicate that with certain parameter configurations, the system exhibits significant subharmonic and chaotic motion. These findings emphasize the importance of optimizing the implant parameters to prevent undesirable aperiodic motion. Optimal design strategies were proposed to map the system's stable parameter region, improving implant stability and auditory compensation effectiveness. These findings demonstrate the feasibility of using shape memory alloys in round-window stimulation to accommodate anatomical variations. The developed mechanical coupling model offers valuable insights for enhancing the design of round-window stimulating implants.