One thing that always struck me as odd about the Parashara smriti is how often it references Manu. The Vishnu Smriti is said by Vishnu, but in response to a desire to know what Kashyapa (reincarnated Manu), also known as Ariṣṭanemi, says. Is any dharma shastra not dependent on him?
This is especially problematic as without speech marks it is really hard to tell how much is quoted.
Even sneakier is the Vyasa Smriti. It makes it really hard to figure out which Vyasa is speaking. However, it does say his hermitage is in Varanasi and the only person that seems to be referenced as having a hermitage at Varanasi is the sun, Mārtaṇḍa, who is one of the Vyasas (specifically the fifth of Vaivasvata Manvantara).
Now, the translation says he remembered from the heart, but the actual Sanskrit wording uses the word for an embryo. So more literally it means he remembered from when he was an embryo. Guess who his father is. It is Kashyapa, tying the Vyasa Smriti, as with all the others examined so far, to Manu/Kashyapa.
Now the Apastamba Dharmasutra (which I doubt as a real dharma shastra but I'll cover it anyway for its popularity) just gives its source as an undescriptive agreement of those who know the law. In the Rig Veda, Kashyapa is described as knowing the law, so it could be him. It's kind of obvious from who they are that some of the authors know the correct law, but that is not the issue.