I can partially answer your question, as I worked on the Sony Heifetz box.
The original 1994 Heifetz set did include the Decca recordings (I worked on
that one also for BMG). The 2010 set was made up of copying the recordings
from the earlier set (minus the Decca recordings), plus bonus material of
the Bell Telephone Hour TV program, and three CDs of previously unreleased
material (my contribution to the set). Some of the recordings from the 1994
set had been remastered since it came out, so in those cases, the later
transfer was utilized.
Not long before the 2010 set was released DG issued the Decca records on a
two-CD set, that included at least one previously unreleased track. My
assumption is that DG did not wish to re-license something to Sony that they
had just brought out themselves.
By the way, the Rubinstein set also has one recording missing from the later
set: the 1964 Moscow recital originally on Melodiya.