Why did Ya'akov switch from "Elokei Avraham" to "Pachad Yitchak"?
See Ramban, who equates this explanation with Targum Onkelos, whereas according to the Ibn Ezra, "Pachad Yitzchak" means 'Yitzchak's fear of Hashem.'
Moshav Zekenim asks that later Yakov did so (refer to 32:10:1:1), and Eliezer said "Elokei Adoni Avraham" (Bereishis 24:42), four times in all (Chizkuni)! Riva answers for Eliezer that after the Akeidah, Hashem swore not to test Avraham again. (Refer to 32:8:4:4 to answer for Yakov - PF.)
Since there is no guarantee that he will not sin before his death. Indeed, Hashem Himself said "Elokei Yitzchak" (above, Bereishis 28:13), because Yitzchak, whose eyes were dim and who was confined to the house, was unlikely to sin. However, Ya'akov was afraid to do likewise.
Tosfos ha'Shalem (3): Alternatively, "Avi" refers to Avraham. (If so, Yakov could have said Elokei Avi Avraham (like below, Bereishis 32:10). Why did he say Elokei Avi Elokei Avraham? The other Meforshim did not address this. Perhaps they hold that 'Elokei Avi' is not called attaching Hashem's name to a person, since Yitzchak's name is not mentioned - PF.)
Why did he use the term 'Pachad Yitzchak'?
Ohr ha'Chayim: Because Yitzchak's primary Midah was Din. 1
Ibn Ezra: "Pachad Yitzchak" means 'Yitzchak's fear of Hashem.'
Pane'ach Raza, Tosfos ha'Shalem (2): Lavan saw in a dream Yitzchak praying with great fear, lest Lavan harm him, and therefore he sent Devorah (to tell Yakov to return). Lavan said (Pasuk 29) "Elokei Avichem Emesh Amar Elai", and not Elokecha - Hashem accepted Yitzchak's prayer, and warned me not to harm you.
Lev Eliyahu (Bereishis p.107): This refers to Yitzchak's fear to bless Yakov, lest he engage in matters of this world and wax rich, and his Shleimus will be blunted. 2
Having accused Lavan of cheating him right, left and center, why did Ya'akov see fit to add that he sent him away empty-handed?
Rashbam: What he meant was that he would not have made him a farewell party, as he claimed he would have done. 1
See above, Pasuk 27.
What does "va'Yochach Amesh" mean?
Rashi: It means that the night before, Hashem had rebuked him (Lavan).
Ramban: It means that He proved him (Ya'akov) right. 1
The same word that Ya'akov used earlier (in Pasuk 37) - "and clarify which of us is right."
רש"י: ויוכח - לשון תוכחה הוא ולא לשון הוכחה: למה פירש רש"י כך?
גור אריה: ה' לא צריך להוכיח ולברר את יגיע כפיו, [ולכן יש לפרש מלשון תוכחה].
למה קרא יעקב לקב"ה "פחד יצחק"?
מהר"ל (נתיבות עולם, נתיב התשובה, פ"ב, עמ' קנב ד"ה ומעתה): מידתו של יצחק הייתה יראה.
מהר"ל (דרך חיים, פ"א מ"ה, עמ' לז ד"ה ויש): מידתו של יצחק הייתה דין, ובעל דין ירא פן יעשה דבר כנגד היראה.