What does "va'Yichros Yehonason Im Beis David" add?
Radak: After Yehonason asked David to have a Bris with his house, Yehonason accepted a Bris with David's house, i.e. his wives and descendants. As long as Sha'ul is king, Yehonason will guard them with all his strength.
Malbim: Yehonason obligated himself to save Beis David, i.e. his wife, Yishai and Yishai's sons, that Sha'ul should not harm them.
What is the meaning of "u'Vikesh Hashem mi'Yad Oyvei David"?
Rashi (from Shabbos 70a): This is a euphemism (really, David will be punished). When David said that the field of Mefivoshes ben Yehonason will be divided among him and his slave Tziva (2:19:30), a Bas Kol said that the kingdom of David's grandson Rechavam will be divided among him and Yaravam.
Malbim: "U'Vikesh Hashem" stands by itself. He prayed that Hashem help him [to protect Beis David].
