Why did Lavan add "Elohei Avihem", and why did Yakov swear by Pachad Yitzchak?
Seforno: He was explaining that he appointed the god of Nachor together with the G-d of Avraham, because he was also the god of Terach, who was the father of them both. However, Yakov swore by the G-d whom his father Yitzchak feared, 1 for Yitzchak was not the son of Terach.
Tosfos ha'Shalem (4): Yakov swore in the life of his father Yitzchak, who did not normally swear, so this was a great matter. 2
Ramban: Pachad Yitzchak is Hashem's Midas ha'Din. Ha'Emek Davar
Why did Yakov swear and not Lavan?
How could Yakov accept an oath in the name of idolatry? "Lo Yishama Al Picha" (Shmos 23:13) forbids this!
Moshav Zekenim: Perhaps Yakov knew that [Nachor] did not serve idolatry (so Elokei Nachor is Hashem). 1
Yakov did not ask Lavan to swear. Lavan mentioned Elokei Avraham and Elohei Nachor on his own. Yakov deviated, and swore by Pachad Yitzchak, to show his disapproval (based on Malbim, Ha'Emek Davar).
If so, when a scribe writes Elokei Nachor, he must intend that it is Kodesh! Lavan said also Elokei Avihem. Terach was an idolater! However, he repented (refer to 15:15:2:1); perhaps 'Elokei Avihem' is Kodesh, based on his final belief. Maseches Sofrim (4:7) and Maseches Sefer Torah (4:4) say that both of them are Chol. Bereishis Rabah (74:16) and other Midrashim say that Elokei Nachor is Chol, and Elokei Avihem is Kodesh (Matanos Kehunah - due to Terach; Etz Yosef - for it includes Avraham), and Chol (PF).