OP, I think you are right in your initial post that to get where MH is coming from, you have to really wrap your head around the idea that progressiveness is an illusion.
Basically, she believes that any given period in history has certain facts around it, that we often have little control over. Things like the economic basis of the society, the technology level, the ability for that society to maintain a complicated bureaucracy, the ability to maintain a complex justice system, or a welfare state.
You also have the basic, unchanging elements of human nature, some good, like self-sacrifice, empathy, familial love, and others not so good, like greed, the will to power, lust.
Within the basic environment, you get a set of social norms, traditions, social structures, laws, and other institutions, that negotiate the arrangements to manage human nature, and also the environment, in a way that allows civil society to function.
And while sometimes they may be clearly negative, more often it's the case that they represent an attempt to find the best balance of competing interests, advantages, and disadvantages. Changes that are meant to be positive in one way often have other disadvantages or trade-offs. And very often you can't entirely mitigate those, you have to make a choice.
One of the big problems with the idea that we are "progressing" to some sort of more and more just or perfect society is that people don't see clearly the trade -offs, and in fact imagine that there aren't any.