Creating a stable Slicer release still requires a lot of manual effort, so unfortunately there is often a rather long time period when a nightly version is ready to be released as stable, yet it is still only labelled as nightly version.
Except a few very specific issues (which should only affect few users), the current nightly version works well, so the potential issues of nightly version are outweighed by those many fixes and improvements that it contains.
This is exactly the plan.
You can still load labelmaps and edit them in Segment editor, you just need to import labelmaps into segmentation node as @cpinter described above.
Per-structure volume management in Editor module was always very limited. As far as I know, per-structure volumes cannot be saved into the scene but you need to merge them into one labelmap before saving. Also, Editor module stores structure names in color nodes (separate from the nrrd files), so if you only save the nrrd files, then your labels are lost.
Since all these issues are solved by the new Segment Editor module (and will not be fixed in the old Editor module), it makes sense to switch from Editor to Segment Editor module. As Segment Editor is much improved in nightly version, it makes sense to switch from 4.8.x to the nightly version.