Why did Ya'akov refer here to Hashem in the singular, whereas in Pasuk 7, the Pasuk refers to Him in the plural?
Sanhedrin, 38b: To counter the Tzedokim, who extrapolate from Pasuk 7 that there are multiple gods.
What exactly, is Yakov saying in the latter half half of the Pasuk?
Oznayim la'Torah: With reference to the Neder that he made on his outward journey to Charan, he is saying that, since the condition that Hasherm would look after him had been fulfilled, he was now obligated to fulfill his part of the Neder.
למה נאמר "לא-ל העונה אותי ביום צרתי"?
מהר"ל (נתיבות עולם, נתיב התשובה פ"ב, עמ' קנב ד"ה ומעתה): מידתו של יעקב שהוא נודר בעת צרה (ותשובה בעת צרה נלמדת מיעקב).