Why did they say "Nekiyim Anachnu"?
Rashi: It is upon you to make the Siman. (If you do not, we are blameless.)
Malbim: The first Shevu'ah was without any condition. Her family could save the entire city, via mixing with them (and then we cannot kill anyone 1 )! That Shevu'ah was void, for we were in your hands [to inform on us to the king]. If one swears to extortionists, it is void (Nedarim 28a).
Perhaps the Isur (to kill people of her family) is considered Kavu'a (fixed), therefore we do not follow the majority (see Sanhedrin 79a). Surely the Shevu'ah does not apply if her family will attack them! Perhaps mixing is different, or Malbim discusses mixing without intent to save others.
Why does it say "mi'Shevu'asech"? We do not find that they swore to her!
Radak #1: Since she told them (verse 11) "swear to me", when they said "Nafsheinu Tachteichem", they swore to her (the Navi recorded the content of the Shevu'ah, but not the entire text of the Shevu'ah).
Radak #2: Since she told them (verse 11) "Hishav'u Li ba'Shem", and they said "Nafsheinu Tachteichem", this was acceptance of the Shevu'ah.
Why does it say "mi'Shevu'asech ha'Zeh"? It should say 'ha'Zos' (feminine)! And why is there a Kamatz under the Tav in "Hishbatanu"? There should be a Chirik (feminine, for she made them swear)!
Radak: The Nekudah changed 1 due to the interruption. After they descended, they changed the condition [of the Shevu'ah] and gave to her a Siman.
Malbim: The masculine denotes strength. [Rachav,] who made them swear, had power and compulsion over them (she could have informed on them). Therefore it was invalid, like one swears to extortionists (Nedarim 28a).
Radak addressed only the latter question.