Why did she give to him milk in place of water?
Rashi, from Targum Yonasan: To see how aware he was, if he would taste the difference.
Rashi (4:19). Milk makes the body heavy, and prone to sleep. Radak - she did not want him to sense when she will strike him.
Ralbag: Milk intoxicates (Yuma 76a).
What is b'Sefel Adirim?
Rashi: It is the flask from which one drinks water; water is called "Adirim" (Shemos 15:10).
Radak citing Targum Yonasan: It is a great flask from which esteemed and mighty men drink. 1
Radak citing R. Yonah 2 : It is the flask of great shepherds, like "Adirei ha'Tzon" (Yirmeyah 25:34).
What butter did she serve?
Radak #1: She gave Chelev Chem'ah (the milk that remains after making butter); it wearies a person and bewilders the mind. The word "Chalav" above applies also to Chem'ah.
Radak #2: After giving him milk to drink, she gave to him butter to eat.
Ralbag: She served fatty milk without removing the cream; it intoxicates more.
