What did the scholars say regarding the authenticity of the mawquf hadiths mentioned by Ibn Abi Shaybah concerning the ruling on eating the cheese of the Majus and the dead animals' rennet contained therein?

It appears that the first chain of transmission is plausible here. The 'an'anah (narration using "on the authority of") of Mughirah from Abu Wa'il and Ibrahim is documented in the Sahihayn (Bukhari and Muslim). As for the second chain, its narrators are those found in the Sahihayn, and the hadith of Al-A'mash is more authentic. The narration from Umar is authentic, whether through the hadith of Mughirah or the hadith of Al-A'mash. However, the narration from Al-Hasan ibn Ali has Jahsh in its chain, and he is unknown in status. The ruling on rennet (infahah) differs depending on its source: if it is taken from an animal that was slaughtered lawfully according to Islamic rites, it is pure. If it is taken from an animal that died naturally (carrion) or was not slaughtered lawfully, there is a difference of opinion regarding it. The majority view it as impure. Abu Hanifa and, in another narration, Ahmad, held the view of its purity, which was the preferred view of Ibn Taymiyyah.