We study single-spin-flip switching paths in a one-dimensional interacting block-spin chain. Previous static work shows that the minimal phase-crossing energy scales as K*sqrt(J) and that any optimal crossing layer has width of order sqrt(J). The missing step toward metastable slowdown is dynamical: one must show that near-optimal switching paths cannot avoid such an extended layer. In this paper we isolate that bridge. We prove that any minus-to-plus path whose maximal energy remains on the near-optimal K*sqrt(J) scale must contain an intermediate configuration with a transition corridor of width at least c*sqrt(J). Thus the static mesoscopic layer is not merely an optimizer of a boundary-value problem: it is dynamically unavoidable along low-barrier switching paths. The present result is a mechanism theorem rather than a full mixing-time theorem, and it prepares the corridor-shell bottleneck analysis needed for later conductance and spectral-gap estimates. Related earlier works by S. Pan are available at DOI: 10.5281/zenodo.19673404 and DOI: 10.5281/zenodo.19689210.
S. Pan (Wed,) studied this question.