Why did Ya'akov offer to work seven years for Rachel?
Seforno: The seven -year wait was to give him time to build up a Parnasah - since the Gemara in Yevamos (65b) prohibits a man from marrying a woman if he cannot provide for her needs. Whereas the work were a 'dowry' which the prospective Chasan would give the girl's father for the privilege of marrying his daughter, as was customary in those days. 2
Moshav Zekenim: Even though the custom is for the girl's father to give a dowry, Yakov said "if you will not give for free, I will work for her, and not transgress my father's command to take a wife."
Tosfos ha'Shalem (1, citing R. Efrayim): An Eved Ivri serves for six years Yakov wanted to work more than this. However, when he worked for money, Hashem did not want to him to work more than an Eved Ivri, and told him to leave after six years.
Rivka said "a few days, until your brother's anger subsides, and I will send and take you from there", and she later sent Devorah to get him (see Rashi on Bereishis 35:8). This implies that he did not know how long it will be! Perhaps Yakov understood that it will be at least seven more years, i.e. 21 from when he left. (PF)
Tosfos ha'Shalem (3): And in seven years she will be an adult, and you will give to her a dowry, like the custom. See Shemos 22:16.
Either way, the time finished in Yakov's 14 years in the Yeshivah of Ever! It seems that his time in Yeshivah is not reckoned with, just like he was not punished for that time. Or, he thought that Rivka limited how long he can stay with Lavan (PF)
Why did Ya'akov see fit to add 'your younger daughter?' How many daughters called Rachel did Lavan have?
Rashi: Anticipating Lavan's attempt to cheat him, he made it abundantly clear that he was working for Rachel his daughter, and not for some stranger from the street of that name, and for his younger daughter, and not for his older daughter whose name he would change to Rachel. 1
Seforno: Refer to 29:18:1:2. Moreover, by that time, Rachel would be ready to get married, and Lavan will have been able to marry off Le'ah.
Why wasn't Yakov concerned lest Lavan change Zilpah's name to Rachel? She was younger than Rachel (Rashi Bereishis 30:10)? She was a Shifchah, and not a proper daughter. Yakov was not concerned for a double ruse (Tosfos ha'Shalem 4). Perhaps Lavan will father another daughter, call her Rachel, and she will be his youngest daughter? If it is clear that he refers to a girl who was already in the world at the time, it should be clear that he refers to the girl who was called Rachel at the time! (PF)
רש"י: הם ימים אחדים שאמרה לו אמו: מנין שזו כוונתו?
גור אריה: אם לא כן, קשה שאמו אמרה לו "וישבת עמו ימים אחדים" וכיצד הוא אומר "אעבדך שבע שנים"?!
רש"י: ותדע שכן הוא וכו': בפסוק כתוב שהימים היו בעיניו כימים אחדים "באהבתו אותה" אבל הם לא היו ימים אחדים באמת?
גור אריה: שבע שנים הם אכן ימים אחדים, אלא שבגלל קושי העבודה שעבד שם יעקב היה ראוי שהם יחשבו לו כימים מרובים, ובגלל אהבתו אותה הם היו בעיניו כימים אחדים על אף הקושי.
רש"י: כל הסימנים הללו למה וכו': לכאורה מספיק לומר 'בבתך הקטנה'?
גור אריה: לבן היה אומר שכוונת התנאי הייתה 'אפי' בבתך הקטנה' והיה נותן לו את לאה, ולכן הוסיף יעקב "ברחל".