The merger of two neutron stars may lead to the formation of a rapidly rotating magnetar. The interaction between the Poynting-flux-dominated jet and the surrounding ejecta gives rise to a pulsar wind nebula, which produces multi-band electromagnetic emission. Unlike previous models that assume constant jet properties, we incorporate the radial evolution of the jet magnetization parameter (σ) and Lorentz factor, where σ decreases and the Lorentz factor increases with radius due to magnetic dissipation. We solve the coupled dynamics of the forward shock (FS), reverse shock (RS), and contact discontinuity (CD), and find that a large ejecta mass (Mej > 3 × 10−4 M⊙) leads to the rapid inward propagation and eventual disappearance of the RS. For smaller ejecta masses, distinct dynamical phases are identified: an initially backward-propagating RS, a prolonged lag of the RS behind the FS and CD during ejecta traversal, and a final convergence after the FS emerges into the interstellar medium. The emission evolves correspondingly, appearing as blackbody radiation from the optically thick ejecta at early times, and later transitioning to synchrotron and external inverse Compton emission from the RS once the system becomes optically thin.