What is the definition of "Shevu'as ha'Alah"?
Rashi, Rashbam, Targum Onkelos and Targum Yonasan: It means 'the Shevu'ah of the curse'.
What does the Pasuk mean when it writes "Yiten Hashem osach le'Alah"?
Rashi: The Torah means that everybody will cite you as an example and say to their disputants 'May Hashem curse you in the manner that so-and-so Sotah was cursed (when her stomach swelled and her thighs dropped - Rashbam)!'
Why does it add the word "veli'Shvu'ah"?
Rashi: To add that everybody will swear by you, saying 'May it not happen to me like happened to so-and-so Sotah!' 1
Why, here by the curse, does the Torah mention the thigh before the stomach, whereas later, in Pasuk 27, when the curse takes effect, it inverts the order?
Rashi: Because that is the order in which the Sotah sinned, first with her thighs and then with her stomach, whereas later it mention them in the order in which they take effect - as the water passes down her body.
Why did Mei Sotah cease even before the Churban?
Ramban: Sotah is unique, 1 inasmuch as it the only case in the Torah which is entirely supernatural. Consequently, it only occurs when Yisrael perform the will of Hashem - in order to prevent Yisrael from copying the immoral lifestyle of the Nochri nations, and to purify them from Mamzerus, so that they are worthy for the Shechinah to dwell among them. This explains why, with the proliferation of sin, Mei Sotah ceased to operate. 2
Refer to Vayikra 14:34:2:2, where the Ramban describes Tzara'as too, as a supernatural phenomenon.