Boundary perturbation methods have received considerable attention in recent years due to their ability to simulate solutions of differential equations of applied interest in a stable, robust, and highly accurate fashion. In this contribution we study the rigorous numerical analysis of a recently proposed high-order perturbation of surfaces method for scattering of electromagnetic waves by a doubly layered, periodic medium in transverse electric polarization. The algorithm in question is a transformed field expansion method which is discretized with a Fourier-Legendre-Galerkin, Taylor series approach. We prove not only results on existence and uniqueness of solutions but also theorems indicating that solutions of our scheme converge to these solutions with high-order spectral accuracy.