Pulmonary artery banding (PAB) is a palliative therapy for infants with congenital heart disease and left-to-right intracardiac shunts, where excessive pulmonary blood flow can lead to congestive heart failure when definitive repair is not feasible or is high risk. Although effective, surgical banding is invasive and often requires repeated adjustments as the infant grows, while emerging transcatheter approaches face complications such as migration, thrombosis, partial collapse, and unpredictable pressure gradients. To support quantitative planning and patient-specific evaluation of PAB strategies, we developed a computational framework that captures the coupled hemodynamics and arterial deformation induced by external banding under physiologically relevant boundary conditions. In our model, blood is treated as an incompressible Newtonian fluid governed by the Navier-Stokes equations and is coupled to a thick hyperelastic arterial wall to capture banding-induced deformation. Physiologically consistent downstream effects are incorporated through a three-element Windkessel (RCR) model, which prescribes a realistic pressure-flow relationship at the outlet. To solve the coupled fluid-structure system efficiently, we use our new explicit, totally loosely decoupled partitioned FSI scheme. This approach advances the fluid and structure subproblems separately (and thus in parallel) while preserving stability for the corresponding linearized problem. To align simulations with clinical measurements and account for uncertainty in key physiological parameters (e.g., proximal resistance, distal resistance, and compliance), we additionally develop a data-assimilation pipeline based on a reduced-order unscented Kalman filter (ROUKF). The proposed pipeline calibrates patient-specific parameters and enables uncertainty-aware estimation of banding-induced changes in pressure and flow. Overall, this FSI-Windkessel-assimilation framework provides a quantitative tool for studying pulmonary flow regulation under external artery banding and establishes a foundation for model-informed, patient-specific treatment planning in neonates with pulmonary overcirculation.