Amud 23b

Two women who were captured - שתי נשים שנשבו

Summary:
The woman is not believed that טהורה אני even if she did not know there are עדים and nevertheless admitted to being captured.

Yes; the רישא and סיפא is where there are עדים and the מציעתא is where there are no עדים, etc. - אין רישא וסיפא דאיכא עדים ומציעתא דליכא עדים

Summary:
According to אביי (even the מקשן understood that) all four cases of the ברייתא are necessary. The רישא דרישא is in a case where איכא עדים.

Why do we also need this; it is the same as the רישא, etc. - הא תו למה לי היינו רישא

Summary:
Case #3 can be derived from either case #1 or #2; however case #4 can only be derived only from case #1.

There is a monetary involvement - דאיכא דררא דממונא

Summary:
There may be more reason to say הפה שאסר if one is a מוחזק than if he is not a מוחזק.

However here, where there is no דררא דממונא; no - אבל הכא דליכא דררא דממונא לא

Summary:
The גמרא could have said that it is necessary to mention the case of העדים שאמרו to negate the view of ר"מ.