Considering that all other redemptions mentioned above are voluntary, why does the Torah make this one an obligation?
Rashi (on Pasuk 48) and Ramban: 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 ben Dodo Yig'alenu"
And why does it add the word "mi'Mishpachto"?