A former IPS officer Dr. Dalbir Bharti writes in his book the constitution and criminal justice administration in the page 16 that
...in a research it has found that out of 2685 verses in the Manusmriti only 1214 verses are original and remaining 1471 verses are interpolated. The researcher has described how the interpolated verses of the Manu Smriti either contravene the views of the Manu as expressed in the other verses or are irrelevant to the subject matter where they are places...
Moreover, Zhuangzi describes about the perfect age as follows,
“In an age of Perfect Virtue, the worthy are not honored; the talented are not employed. Rulers are like the high branches of a tree; the people, like the deer of the fields. They do what is right, but they do not know that this is righteousness. They love one another, but they do not know that this is benevolence. They are truehearted but do not know that this is loyalty. They are trustworthy but do not know that this is good faith. They wriggle around like insects, performing services for one another, but do not know that they are being kind. Therefore they move without leaving any trail behind, act without leaving any memory of their deeds.”
Going by this description of Satyuga, many laws like of killing, theft, inappropriate sex, etc. shouldn't simply exist because of the absence of evil. In that way, Manu Smṛitī being Smṛitī of Satyuga doesn't square well. Even Lao Tsu says the same,
When the Tao is lost, there is goodness.
When goodness is lost, there is morality.
Lao Tzu also meant that in a perfect age, a man is simply righteous but ain't aware of his qualities. It's only when unrighteousness appears in abundance that there is realization of righteousness existing different from unrighteousness.
So, do we have any research paper or any content which shows or argues how Manu Smṛitī is interpolated. And possibly what are those possible verses if interpolated according to the researches.