We develop a microscopic theory for how slow Liouvillian sectors are selected in an open correlated lattice. The starting point is not a postulated non-Hermitian band, but a local interacting resonance between an on-site doublon and a branch-resolved nearest-neighbor bond. This resonance selects a complete doublon-bond shell whose doublon component controls reservoir visibility and whose orientation-resolved conversion amplitudes control shell mobility. Projecting the microscopic hopping onto the selected shell yields a branch-selective dimerized channel. In the dilute regime, a boundary doublon-loss channel yields an exponentially slow edge-memory pole through a Zeno-type return. At the shell-critical point, the edge pole is replaced by a near-zero standing-wave doublet with an algebraic coherent spacing. At finite shell filling, a projected continuation of the same parent shell supports number-conserving phase locking on nonoverlapping dimers. Under an additional population closure, the resulting pinned-boundary defect generator yields reciprocal diffusion and nonreciprocal boundary accumulation. The resulting mechanism identifies the reservoir-engineered fast block as the selector of the observable slow sector, while the microscopic parent shell remains fixed.