In your example, the role of intuition becomes problematic if laws were based on it. Except that such intuition won't turn into a law unless a majority of people adopt it, which generally won't happen if it's not plausible enough. Although it could happen, and I agree with you that it could become dangereous, if reason and experts' opinions stop being highly valued by society.
> Should we make laws on the presumption that moral nihilism is true, or that it is false?
How should I interpret the "Should" in this question, as "In order to be consistent, should we make laws...", or as a moral obligation :) ?
The underlying assumptions in your question are 1) that there should be a single principle based on which we make laws, which is not necessarily true, that's why we end up with all kinds of inconsistencies, 2) that lawmakers care about philosophy when making laws, which doesn't hold for me.
I'm not well versed in philosphy, so I'm arguing here from my basic understanding of things. Laws are practical. What would be the practical consequences if laws were made on the presumption that moral nihilism is true?