To answer both questions with one blithe statement: I don't think we do.
Now, that might not be the most useful answer to your question, but I'm speaking as one of the people tasked with attempting to apply this stuff, and largely speaking, keeping the wheels turning. Trying to define solid rules on quorum and then how opinion-de-jour gets applied to the site, well it leads to madness and frustration.
My observation of what tends to happen is:
- Person has opinion 💡
- They either apply it without asking and somebody complains on Meta, or
- They ask whether or not it's right on Meta
- More opinions get shared and people deploy votes
- The general outcome usually get's applied to that case.
That sets a precedence so the next time another person has the same opinion and tries to act on it, we can reuse the old arguments. But similarly often, the thing gets churned up because the context is different, or the outcome wasn't satisfactory. I think that works for us.
I honestly think it's dangerous to try and define a quorum here, or solid finite rules on our democracy. There's always situational context. There's nothing to say that somebody might one day, with hindsight maybe, have a better opinion. Speaking as one of the 48% of Brits being told that democracy is apparently immutable (unless it's trying to shove the same crappy deal through Parliament) I'm quite passionate about this concept of encouraging the revisiting old decisions.
Basically, if you want to do something you suspect people disagree with, make your best case right there. By all means cite the previous discussions but they're not law. If you have a better reason on the day, there's no reason you might not buck the trend.
Just to pull this back to me, I'm not always here. Few mods are. There are a great many high-rep users who do a fantastic job of answering Meta questions so that we don't have to. If you see one of us doing something that's against a recent consensus, please bring it up. We might have a good reason, we might not and we might need to revisit our actions.