Why did Ya'akov see fit to mention the fact that Avraham acquired Me'aras ha'Machpeilah?
Seforno: Because they had spent many years in Egypt, and with the passing of time, were likely to have forgotten the details concerning its acquisition from Efron and the Bnei Cheis. Moreover, he added, both he and two generations before him had buried their dead there and nobody could possibly object when they came to bury him there. 1
Da'as Zekenim, Hadar Zekenim: Yakov clarified all possible uncertainties - in which field is the cave; no one will protest; for Avraham bought it; he bought it from the Nasi (Efron), so the sale cannot be contested; and it was sold as a burial-plot. They cannot say that no Chazakah was made, for they buried Avraham there. They cannot say that they could not protest against their Nasi, for he bought 'me'Es Bnei Ches."
Why did he add the words "la'Achuzas Kaver"?
Ramban: To teach us that it was Avraham who issued the command to maintain the Me'aras ha'Machpeilah as a permanent burial-plot.
Rashbam: Since it was sold as a burial-plot, nobody could possibly object when they came to bury him there. 1
And why, when they actually buried Ya'akov, does the Torah repeat it all over again (See 50:13)?
Ramban (on Pasuk 31): The Torah is coming to teach us that Avraham purchased the cave on behalf of the three Avos exclusively, and when Ya'akov was buried, the issue was closed and nobody else was allowed to be buried there. 1
למה הוסיף יעקב "במערה" אחר שאמר לעיל "אל המערה"?
גור אריה (לעיל,כט): כדי שלא יחשבו האחים שצריכים להביא אותו רק סמוך למערה- הוסיף יעקב "במערה".