What did the seven sheep come to prove?
Rashi: Nothing! They were merely to substantiate that the well belonged to Avraham. The real proof lay in the fact that the water rose to meet Avraham, following the dispute between the shepherds of Avraham and those of Avimelech. 1
Seforno: They came to seal the agreement between the two parties, following Avimelech's admission that Avraham's shepherds had dug the well. 2
Targum Onkelos and Targum Yonasan: The gift would serve to induce Avimelech to testify that Avraham had dug the well. 3
Moshav Zekenim: People always take sheep to the well, to drink. People would ask about them, and the shepherd would say that they testify that it is Avraham's well. He gave females, which people keep (for babies; they slaughter males). He gave seven, for it resembles the word "Shevu'ah".
Much in the same way as we find in Rus (4:7), where 'One man took of his shoe and gave it to his friend' - to substantiate the agreement between Bo'az and P'loni Almoni.
According to Rashi and the Seforno, it was the sheep or the covenant that would achieve that.
Why is "Kevasos" written without a 'Vav'?
Moshav Zekenim (on verse 28) #1: It suggests the singular, for they all looked alike.
Moshav Zekenim #2: It is lacking to hint that it was improper for Avraham to make this Bris.