Split-prime supercongruences at a mixed CM point: order drop, Eisenstein dichotomy, and reduction to a unit-root congruence

Symmetric-cube hypergeometric coefficients at the mixed CM point exhibit a clean arithmetic dichotomy governed by splitting in the Eisenstein decomposition of the logarithmic derivative. At split primes both Eisenstein pieces share the same local factor and the tower survives; at inert primes a unit-root obstruction kills it. We prove by explicit counterexample that the natural bivariate congruence on the full Eisenstein module is false, while the congruence survives on the diagonal determined by the mixed combination. By descending the defect forms to the Fricke quotient, where the two-cusp obstruction disappears, we reduce the full split-prime supercongruence to six explicit scalar congruences on principal parts, verified computationally for split primes up to 31.