Here's the crux. We currently decide both values and facts by deciding for actions and policies, and decisions using popular opinion will therefore make errors

Here's the crux. We currently decide both values and facts by deciding for actions and policies, and decisions using popular opinion will therefore make errors about facts. How we negotiate facts and values should be different. We could define what our values are, decide if it is just to use potentially violent force to enforce laws that forbid substances that have detrimental health effects and flesh out what we mean by a detrimental effect, what the threshold of that is for different age groups -- these are things that are pretty anthropic, can't be decided otherwise. We can then collect facts about which substances fit into our decided categories, and avoid making mistakes that fail to serve the values we chose. If (because the fact process, being human, is imperfect) we find new values for facts, we can adjust policy and actions without readjusting the values side of decision making.