I'm sure that DG is getting a bit nervous over Abbado's state of health. They seem to be releasing a whole slough (slew?) of strange odds and ends with him.
I'm looking at this very differently, however true your first surmise might be: It's a whole set of new projects going on with Abbado & DG, perhaps prompted by the conductor's "second coming" that by now has brought to us such rarefied performances in the last few years. For instance, his recordings with Orchestra Mozart and Giuliano Carmignola are definitely something entirely new for him (for all the parties included). Note that we now have 4 full CDs' worth of music from this period instrument (sic!) venture (the Mozart violin ctos, a set of symphonies, and the better known sinfonia concertante). They are also touring quite a bit, which makes me think of it all as less of a birthday homage and more in line of a fresh project. There is also stuff in the pipeline that he has recently recorded with Dudamel's Venezuelan players (a newly done Symphonie Fantastique!) plus some more that I don't remember right now. Quite a bit of activity, in fact, I'd say, considering. He also seems to be quite busy in the concert circuit again.
I think you may be referring to that "Marches and Dances" issue which may or may not be a re-issue of some past compilation, though I have really no idea. That does seem to be kind of a one-off odds-and-ends type of an issue.
They should be looking at people like David Robertson; Robert Spano; Donald Runnicles; Joann Falletta; Marin Alsop (happily cranking out stuff for Naxos), etc.
In this connection I always trumpet the merits of the second favorite
American* U.S.-born conductor of mine: Kent Nagano (has also been recording for DG btw [or at least their "20/21" imprint]). (David Robertson is one and Dennis Russell Davies may be a third.) But isn't Harding a true Brit (last I heard), Alex?
PT
*Dudamel for one is "American" as well.