Why did Yaakov not request the older daughter?
Tosfos ha'Shalem (2): He realized that Leah was destined for Esav. He thought that if he marries her, Esav will be angry also for this.
Tosfos ha'Shalem (17:2): Rivkah gave to him a sign, that his Zivug would resemble her.
Refer to 29:18:151:2.
He cried when he saw Rachel, for he saw through Ru'ach ha'Kodesh that she would not be buried with him (Rashi to 29:11). I.e. he knew that she is his Zivug! (PF)
The Torah calls Lot's daughters Bechirah and Tze'irah (19:31). Why does it call Lavan's daughters Gedolah and Ketanah?
Tosfos ha'Shalem (3): Some of Leah's descendants will get great (permanent) gifts, of Kehunah and kingship. Rachel's descendants (Yosef and Sha'ul) received temporary kingship.
Ohr ha'Chayim: Lavan's daughters were intrinsically old and young, not merely with respect to each other, e.g. 20 years old and five. There was no way that Lavan could claim 1 that Rachel was the Gedolah. 2
However, the Rosh (to 29:18) says that Lavan later called Leah 'Rachel' and said that she is Ketanah, i.e. shorter; "Yefas To'ar" is [tall] stature. This is why later (29:26), he said "here, we do not give Bechirah before Tze'irah," i.e. older before younger.
Ohr ha'Chayim: Also, "ul'Lavan Shtei Vanos..." teaches that it was known that Leah was the older, so Lavan could not claim oppositely. It also mentions the stark difference in appearances, so he could not switch them.