Abstract: Pauli spin blockade (PSB) readout in silicon quantum dots relies on spin-dependent charge transitions between two-electron states. While conventional descriptions often assume a single-orbital model, strong electron–electron interactions, valley degrees of freedom, and alloy disorder can significantly modify the relevant tunneling processes. We investigate how Wigner-molecular correlations and valley physics influence the singlet and triplet tunnel couplings relevant to PSB readout. Using full configuration interaction, we construct a low-energy effective Hamiltonian and extract the singlet and triplet tunnel couplings and the exchange splitting at (0,2) charge configuration. We complement these numerical results with an analytical description connecting tunneling amplitudes to spatial wavefunctions and valley phases and examine how alloy disorder affects these effective parameters. This framework provides a microscopic description of spin-dependent tunneling beyond the conventional single-orbital model.