Why did Hashem find it necessary to promise Yaakov that He would accompany him?
Malbim: He promised that He will be with him in Chutz la'Aretz, just like in Eretz Yisrael.
What does "Ki Lo E'ezavcha" mean (that is not contained in the preceding promises)?
Rashi (to 28:20): It means that Hashem would sustain Yaakov. 1
Rashbam: It refers to Yaakov on his travels, as a person who is traveling needs special protection. 2
Since this was the first time that Hashem spoke to Yaakov, how could He state "Asher Dibarti Lach"?
Rashi #1: "Lach" following an expression of Dibur means, not 'to you,' but 'about you;' and Hashem did indeed promise Avraham (in Bereishis 21:12) that He would establish Yaakov as his (Avraham's) heir. 1
Rashi #2 (to Bereishis 32:13): As in the previous explanation, it refers to the promise to Avraham that his children will be like the stars of the Heaven and the sand by the seashore (to which the Pasuk refers there, whereas here Hashem promised him that they would be like the dust of the earth).
Rashbam: It refers to the previous phrase "and I will return you to this land," Hashem is assuring Yaakov that He will not forsake him until He has fulfilled what He just promised him. 2
Seforno: It refers to the earlier phrase " ... and you will spread out in all directions." Until that materializes, Hashem is assuring him that He will carry out all the promises in this Pasuk and that He will not forsake him.
Why does it say twice Asher - "Ad Asher Im Asisi Es Asher Dibarti Lach"?
Hadar Zekenim: This is an expression of a Shevu'ah. Yaakov also said "Asher" twice in his vow.
What is "b'Chol Asher Telech"? Yaakov was going only to Charan!
Ha'amek Davar #1: It is in all your ways in the world - whether in wealth or poverty, you will be guarded.
Ha'amek Davar #2: This promise was primarily for his seed - whenever they will be scattered. Hashem promised him a special Hashgachah to return (him or his seed) to Eretz Yisrael - but then, the Hashgachah is according to the generation's deeds.
QUESTIONS ON RASHI
Rashi writes: "'Im Asisi' - [In this case,] the word 'Im' means 'Ki' (that)." What does Rashi mean?
Gur Aryeh: The word "Im" usually means "if;" but Hashem's promise here was certain and unconditional! Here, the word means 'Asher' (which is one of the meanings of 'Ki').