Don't forget the space habitats have a technology, so far unexplained, which managed to put Ardent and Gavia on earth without apparently any vehicle at all, and Gavia expected it also to retrieve them as well, tho' it hasn't. Not unreasonable, I submit, to assume they could be retrieved from a rocket in random orbit.
My impression is that the twins were sent down to Earth to do a mission, and if they can't get that mission done, there's no reason for the Praeses to go to any effort to retrieve them.
This would be true even if there were a working rocket system. Since it seems unlikely that transport between orbital habitats uses anything a crude as shuttles, there probably are no docking ports on them that the rocket could interface with. So unless the Praeses have hitherto-unseen warm fuzzies for their two spaceborn tools, there's still no getting back even if you could put the pair into a compatible orbit.
A word about orbit. It's not really that difficult to get up past most of the atmosphere; all you need is a large enough helium balloon. But to get to orbit, you have to go
really fast, usually in the same direction as the Earth's spin. That's why they generally use rockets to get up there.