You are right. The Bible is not about metaphysics and much of our problem comes from confusing Christianity with metaphysics.
There is where moral progress comes in play. Democracy is the wil of the majority to a certain degree, some basic humans rights are granted by the constitution of the country and cannot be violated even if the majority choses to.
Yes and no. At least for the US Constitution, all of it can be rewritten through the amendment process. In fact, one could call a constitutional congress and chuck the entire constitution out the window. We keep those protections in place because we don’t trust ourselves, in a sense. We recognize that the majority can trample on the rights of the minority, and that any majority can become a minority at any time. Therefore, we set out rules ahead of time that we adhere to even if it is inconvenient to the majority. It’s a bit like not allowing the rules of a sport to be changed by the leading team during a match.
Yeah, but we do recognize these things as morally valuable, which is a point in favor of mankind’s morals.