Why did Rachel and Leah say "due to these reasons, we should follow Hashem's command"? And why did Yakov need to give reasons?
Seforno (Pasuk 14,16): They explained that it is not hard for them to leave, and that he should obey Hashem's command and leave immediately, without asking permission from their father.
Lev Eliyahu (Bereishis p.112): One should not think that he must suffer here to serve Hashem, and then the world to come will be nice. Rather, he should see that Avodas Hashem is best for him also in this world!
What does "Hitzil" mean?
Ohr ha'Chayim: It refers to recovering from a thief. He took our property (and Hashem returned it).
Why did they say "ul'Vaneinu"?
Ha'Emek Davar: What should have been ours, Hashem gave to you. There is no difference, for in any case, after [we die] it will be to our sons. There were not concerned lest Yakov marry another wife and father more sons (like Lavan suspected, and made Yakov swear
He should say that they knew that Yakov will have 12 sons through four wives (Rashi, 29:34)! If not, Yakov could find a wife elsewhere; Ramban (38:2) says that his sons married girls from Mitzrayim, Amon, Mo'av, Yishmael and Bnei Keturah! Had Rachel received from Lavan, her [one or two] son(s) would get her entire share. Now, most of it will go to Yakov's 10 other sons! (PF)