Well, first, I’m not happy with two entity-types for series. We see that this design gives place for discussions (again) – anyway without a more detailed guidance.
IMHO it would be better to have just one entity-type (and the kind of relationship, Release group or Release, just as attribute). With this it would also be easier to change relationships for an album.
However, the DB schema is like it is.
It is also influence by the way we see release groups, i.e. what should be grouped together and what not. Also other ways of doing it might be possible (e.g. with support of new relationships for loose linking of release groups).
I’m not a friend of redundant information and just for this reason I don’t like to have it as Release group series and as Release series.
As you, @psychoadept, mentioned, later releases can break the integrity of a Release group series. This is maybe not nice at all, but maybe acceptable. [For me the original releases are typically more important.]
Since the Reclam Musik Edition series is already a group of re-releases (typically of best-of-compilations), I guess it’s the better way to have it just as release series. This re-releases have a very tight scope (own title, own cover, focused to Europe) and such a series should not bear on other, earlier (and also later) releases of the same album/compilation. In other words: With linked Release group series for “Reclam” I would not merge such a “All Time Best” release group with the original release group.