This paper reports the V42A–V42C validation sequence for a descriptor-scaled ECSM molecular target-response law. V42A applies the frozen V41 atom-count response law, without retuning, to isoflurane and trichlorofluoromethane (CCl3F). Isoflurane provides a clean same-grid third-target validation and passes the strict threshold, reducing the key held-out mean residual from 0.9768 for bare ECSM to 0.2143. CCl3F improves strongly relative to bare ECSM, reducing the key residual from 1.8571 to 0.4093, but remains above the strict 0.25 threshold. V42B tests a simple heavy-target-burden descriptor repair for CCl3F and finds only marginal improvement. V42C tests a stronger descriptor-coupled scale-law repair, selected on methane, sevoflurane, and isoflurane while holding out CCl3F. This improves the CCl3F key residual to 0.3554 but still fails strict recovery. The final conclusion is that the V41 descriptor-scaled ECSM molecular response law generalises cleanly to isoflurane, while CCl3F defines a heavy-halogen boundary for the current descriptor class. The final label is PASS_MODERATE_V42C_IMPROVES_CCL3F_NOT_STRICT.