Question

Hello Rabbi,
I am trying to understand a section in the Mishneh Torah.
Book of Holiness, chapter 3, section 13:
And for this reason, the law requires that any milk found in the possession of a gentile is forbidden, lest he mixed into it milk from an impure animal. But the cheese of gentiles is permitted, because milk from an impure animal does not curdle. However, in the days of the sages of the Mishnah, they decreed concerning the cheese of gentiles and forbade it, because they curdle it with the stomach lining of their slaughtered animals, which are carrion.a0
Book of Holiness, chapter 4, section 19:
The rennet of a carcass and the rennet of an impure animal is permitted, because it is like the rest of the filth in the body. Therefore, it is permitted to curdle cheese with the rennet of an animal slaughtered by a gentile, and with the rennet of a domesticated or wild impure animal. But the skin of the stomach is like the rest of the intestines, and is forbidden:
Questions:
1- What is the difference between the stomach and the skin of the stomach?
2- If it is permitted to curdle cheese with the rennet of a carcass or an impure animal, why did the sages decree against it because they curdle it with the skin of the stomach?
3- Does this mean that the skin of the stomach is when they take a piece of meat and put it into the vessel of milk, whereas the stomach itself is permitted when it is used as a vessel for curdling cheese?
Thank you,
Abrahama0