What was Lavan referring to when he said "Malei Shevu'a Zos"?
Rashi, Targum Yonasan and Rashbam: He was referring to the week of festivities (Sheva Berachos) 1 that Yaakov was to complete before marrying Rachel.
Ha'amek Davar: First finish "Shavu'a Shanim," seven full years from day to day (not merely "Sheva Shanim," which is only until Rosh Hashanah of the seventh year), and then you will get Rachel. We did not stipulate about Leah, but I see that you are willing to work seven years for a wife, and you cannot get Rachel before marrying Leah. This is like the Ramban; refer to 29:21:1:4.
See Ramban, who refutes this on the grounds that Sheva Berachos is an enactment of Moshe Rabeinu, and was not yet instituted in the time of Yaakov and Lavan, though he does concede that it may have been customary to perform it, particularly according to the Yerushalmi (Mo'ed Katan 1:7), which learns from Lavan that we do not mix two Simchos together.
Why does it say "v'Nitenah Lecha," and not "Eten"?
Rashi: This is the plural (we). 1 Ramban - the people, who would not allow me to marry off Rachel before Leah, would now agree to give him Rachel.
Tosfos ha'Shalem (8, citing R. Efrayim): "V'Nitenah" is passive. It is as if she is already given to you. 2
Tosfos ha'Shalem (1, citing R. Efrayim): We will make a big party for Rachel, like we did for Leah. 3
Tosfos ha'Shalem (9): All people of the city will be guarantors for this.
Malbim: It is past tense (Nif'al). Leah is already given to you on credit, for another seven years that you will work. What you already worked was for Rachel. If you do not want to keep Leah, you must wait a long time for someone else to marry her!
Gur Aryeh: We might have thought that the introductory "Nun" indicates the passive (Nif'al) (meaning, 'and she will be given to you'). Some Mefarshim differ with Rashi; see further.
According to the Ibn Ezra, "v'Nitenah" is Nif'al (passive) - 'and there will be given to you.'
Why did Yaakov immediately agree to work for another seven years for Rachel?
Why did Lavan say "Gam Es Zos"?
Tosfos ha'Shalem (10): These are two inclusions, to include Bilhah and Zilpah. 1
He already gave Zilpah! Perhaps this is a hint that later, he will marry both of them. (PF)
QUESTIONS ON RASHI
Rashi writes: " v'Nitenah - in the plural... 'And we shall give...." Why did Lavan use the plural?
Gur Aryeh #1: This was a respectful way of speaking, to include the invited guests who would escort the Chasan and Kalah.
Gur Aryeh #2: Lavan claimed he had switched the bride due to the local custom, saying, "such is not done in our town" (29:26), as if it was the townspeople who would not allow it. He therefore used the plural here as well, "We will let you marry...." 1
If so, this choice of words was another part of Lavan's deceit.