1)
Whom may they not force to work?
1.
Radak: This is like it explains, "ha'Ivri veha'Ivriyah" slaves.
2)
What is the meaning of "l'Vilti Avad Bam bi'Yhudi Achihu Ish"?
1.
Radak: The words must be re-arranged. It is as if it says l'Vilti Avad Ish bi'Yhudi Achihu. Also "Mekom Sham Kever" (Yechezkel 39:11) means Mekom Kever Sham, and other verses in which what belongs earlier is written later.
2.
Malbim: This was a second enactment. They enacted (a) to free slaves who did not yet work six years, and (b) In the future, one may not have an Eved Ivri.