Remember that I'm an anti-moral realist trying to steelman the moral realist position. As an anti-realist, it is not surprising at all that moral reasoning changes. I think there's no particular reason to think that the scientific method (or some moralistic equivalent) is available to "discover" moral truths. But the moral realist has great difficulty explaining explaining quasi-random movement in morals.

Anyway, we seen to be disagreeing on the meaning of the words "progress" and "regress." To illustrate: Imagine a plantation manager, overseeing a huge plantation. Usually, the plantation grows enough food to give everyone on the plantation an adequate diet. The manager apply his moral theory and decides to actually feed everyone an adequate diet.

Now an external event causes the plantation to grow insufficient food for the people living there. The manager applies the same moral theory and decides to feed some people an adequate diet and some an inadequate diet. Under one understanding of regress ("regress1"), this change is moral regress. Under another understanding ("regress2"), the change is not moral regress, merely changed circumstances.

You seem to be talking about moral regress1 and scientific regress1, while I am talking about moral and scientific regress2. I would argue that regress2 (and its counterpart, progress2) is the concept generally meant by ordinary usage. Further, regress2 is the more useful definition in a meta-ethics conversation, because regress1 is not usually evidence for or against moral realism.

Assuming moral realism, the human mind may not be designed to discover truth about morality any more than it is to discover turth about any other aspect of nature, it is designed to be as adaptive as possible

But the central feature of most object level moral theories is that acting morally is more adaptive. For a utilitarian, acting morally generates more utility than acting immorally. Given the benefit of hindsight, shouldn't we notice when our society is generating less utility than it could? And thus act to change our behavior towards generating more utility. That's why I think that the existence of moral facts would constrain the behaviors of individuals. If we can't detect whether more utility is generated, then there's no reason to believe in the existence of universal and objective moral truths.

As an anti-realist, I take the position that "generating less utility than society could" is not a well formed assertion. But the moral realist does think the phrase is universally meaningful.