Considering that all other redemptions mentioned above are voluntary, why does the Torah make this one an obligation?
Ramban, Moshav Zekenim (47): To prevent him from becoming attached to his master and emulating his deeds.
Why does the Torah add "O mi'She'er Besaro"?
Torah Temimah (citing the Toras Kohanim): To teach us that the closer the relationship, the higher obligation to redeem the Eved Ivri. 1
Torah Temimah also cites the Rambam, who learns this from the previous phrase "O Dodo O Ven Dodo Yig'alenu."
Why does it add the word "mi'Mishpachto"?
Torah Temimah (citing Yevamos, 54b): To confine the redemption to the Eved Ivri's father's family. 1
Because the mother's family is not called 'Mishpachah' (See Bava Basra 109b).
Kidushin 15b learns redemption via strangers from "v'Nig'al." We also learn from this that he is redeemed totally, and not partially (ibid., 20b)!
Moshav Zekenim citing Ri: Really, it teaches redemption via strangers. I could exclude partial redemption from "Yig'alenu."