if you start with the premise that there’s no underlying guidelines to circumscribe the sheer brute power of legal authority in any legal regime.

That is certainly not what I think. Rights do not spring from nowhere: They derive from certain moral values that are commonly shared within a society - such as that murder is wrong. There is no absolute morality however, as witnessed by the fact that slavery was once considered moral, but is no longer.
It is only once moral values are enshrined in law that they become rights and cease to be only opinions.