Bulk hexagonal diamond has been synthesized by independent routes, but its structure remains contested: the two recent refinements disagree even on the sign of the difference between its two inequivalent bond lengths, 238 mÅ apart, and both depart from an earlier 2003 refinement. Here we test the competing structures with first-principles lattice dynamics. Relaxed hexagonal diamond has an interlayer bond longer than the intralayer bonds by 24 mÅ in both functionals, an effect of its eclipsed conformation that scales with polytype hexagonality. The bright zone-center A_1g mode gauges the interlayer bond at ≈-2,100~icm Å⁻¹, and neither refined coordinate reproduces the full pattern of measured modes. The only structure matching the twinned sample's three bands requires tens-of-gigapascals confining stress and lattice constants excluded by its own diffraction. Raman spectroscopy and diffraction jointly select a small positive bond asymmetry: inverting the spectrum of the phase-pure sample gives OB-OA=24±3~mÅ (95% interval), and two determinations on separate samples give +33±8 and +60±45~mÅ. The 1,529 feature cannot be assigned to homogeneous ideal 2H diamond, and the local HRTEM observation remains an open puzzle.