Because it would be a political decision. Which means they'd need corrupt military higher ups to agree to it and be bribed. Why send Chang at all when he actually seems to be a good man is beyond me.
They needed someone to testify as to the "horrors" of Ceres who has a reputation of being credible and honest. Rhonda is neither. He's the "carrot." Rhonda is the "stick." My knowledge of "spooks" goes by Tom Clancy and Dirk Pitt type novels.
The current plot involves the UW trying to "find" an incident which they can publish back home about how "lawless" the belt is so they could justify a full scale invasion, without revealing the fiscal crisis brought about by the "let's appease the welfare crowd" form of government the UW has.
Unable to find one, which Chang would repeat at a press conference, Rhonda is trying to provoke one, by the most underhanded tactics imaginable. Her primary mistake is doing so in front of Chang. Bound by orders or not, it was a DUMB and arrogant move on her part.
Ed's biggest mistake here is not demanding proof that Jenny is still alive. Rhonda has already shown that her word can NOT be trusted. While I believe the threat, subordinates who are
unbalanced enough to follow those orders tend to be rather impatient when their "toy" is dangling before them. So if Rhonda's threat is true, how do we (or Ed) know that Jenny hasn't ALREADY been gang raped, and cut into little pieces. Rhonda's probably thinking "after we broadcast the 'beating' we can just say young Jenny was 'yet another victim of belter lawlessness' and send her mutilated corpse to the 'grieving family.'" I just hope Ed is stalling here because his buddies need time to find and rescue her, and it's not already too late.
Edit: Don't forget, the "president" of the UW has already shown that he's willing to IGNORE/DEFY his government's own laws when they don't do what he wants. "We need a new legal department.... Smarten up, this needs to be done on the QT."