The observation that such an independent disvalue would be convenient doesn't influence whether treating it as such would accurately represent existent human values

I agree, but it influences the optimal research strategy for finding out how to accurately represent existent human values. The fact that pain being an independent disvalue would be convenient implies that we should put a significant effort into investigating that possibility, even if initially it's not the most likely possibility. (ETA: In case it's not clear, this assumes that we may not have enough time/resources to investigate every possibility.)