Causal Electromagnetic Response in ECSM: Corrected Propagation, Microscopic Closure, Nonlinear Signatures, and Observational Survival

This work constructs a corrected and observationally constrained electromagnetic sector for the Emergent Condensate Superfluid Medium, or ECSM, framework. A homogeneous scalar multiplier of the Maxwell kinetic term is shown to modify field normalisation and wave impedance without independently changing the Maxwell light cone. Genuine finite-response propagation instead requires separate electric and magnetic constitutive functions, written as Z_E(omega) and Z_B(omega), giving: n squared = Z_E divided by Z_B and Z_wave divided by Z_0 = 1 divided by the square root of Z_E times Z_B. The constitutive functions are derived from gauge-invariant auxiliary polarisation and magnetisation modes coupled to electric and magnetic fields defined relative to the local medium response state. Their exact oscillator dynamics recover Maxwell electrodynamics and Lorentz-compatible propagation in the coherent low-frequency limit, while their overdamped low-inertia limit produces a causal single-pole memory kernel. A complementary heavy-scalar closure generates a nonlinear electromagnetic operator and predicts background-field birefringence and third-harmonic response. The nonlinear coefficient is translated into SI laboratory variables using notation that explicitly distinguishes electromagnetic kinetic normalisation from the physical SI vacuum impedance. The exact six-parameter response is tested through a deterministic logarithmic scan of 100,000 points against LHAASO GRB 221009A differential photon propagation, GW170817 and GRB 170817A multimessenger timing, resonator isotropy, and a conservative benchmark extra-opacity requirement. Of the tested points, 19,417 satisfy all imposed envelopes. The surviving region is strongly restricted. Appreciable tracked response fractions require extremely weak damping and high inertial turnover, whereas ordinary finite damping is compatible with observation only for extremely small tracked fractions. Within the tested closure, the simultaneous combination of appreciable response, ordinary damping, and a turnover near currently observed photon energies is excluded. The paper explicitly corrects an earlier ECSM identification in which the effective propagation speed squared was set equal to 1 divided by a common scalar response factor. The emergent gauge construction, coherent Maxwell limit, and finite-response medium interpretation are retained.