Considering that all other redemptions mentioned above are voluntary, why does the Torah make this one an obligation?
Ramban and Moshav Zekenim (on Pasuk 47): To prevent him from becoming attached to his master and emulating his deeds. 1
See Ramban DH 'Vetzivah ha'Kasuv'.
Why does the Torah add "O mi'She'er Besaro"?
Sifra: To teach us that the closer the relationship, the greater the obligation to redeem the Eved Ivri. 1
See Torah Temimah, note 256.
Why does it add the word "mi'Mishpachto"?
Yevamos, 54b: To confine the redemption to the family of the Eved Ivri to the father's side of his family. 1
Why does the Torah write the word "Yig'alenu" three times in the current two Pesukim (See Torah Temimah, note 259)?
Kidushin, 21a: To teach us that all Ge'ulos (Batei CHatzerim, adeh Achuzah and Nimkar le'Yisrael
What are the implications of "O Hisigah Yado"?
Sifra: It implies that he obtains the money to redeeim himself by his own efforts and not by means of a loan.
What are the implications of the word "Venig'al"?
Kidushin, 15b: It means that he goes free, irrespective of who redeems him. 1
Targum Yonasan and the Sifra: It means that, if neither one of his relatives nor he himself redeems him, then Beis-Din (all of Yisrael) is oblogated to do so.
The Gemara in Kidushin 15b learns redemption via strangers from "ve'Nig'al". But the Gemara there on 20b, also learns from "ve'Nig'al" that he must be totally redeemed, and not partially?
Moshav Zekenim (citing the Ri): Really, it teaches redemption via strangers, and we preclude partial redemption from the word "Yig'alenu."