RPO is not a static operation Edit: to expand, that article you sent says that gssap conducts RPO meaning it moves along the belt as you mentioned. Since it is so close it can actually look from more angles in a safer manner than a retro orbit

"More accurate tracking and characterization of man-made orbiting objects." You can do a lot more at ~0 relative velocity versus ~6 kps though

Well above or below wouldn't work. If you mean further out then that'd be a bad idea since that's where most graveyard orbits are and if you mean closer in then sure that's possible but positions of GEO satellites do not require space-based detection. There is really no point in doing this unless you were operating like gssap

Doing it in retro would make it near impossible to safely catalog anything. They just want to have the threat of it being there looming.