In the first system I move the "long" InstrumentName of the group
with \override InstrumentName #'self-alignment-X = #-0.5 to the
left, so it doesn't collide with the staff names which I want to
have in the first system. Later I only want to have the group name
there, but in alignment with the staff names of the
"non-group"-staffs. Unfortunately InstrumentName and
ShortInstrumentName cannot be modified independently. Otherwise I
would have left the contents of InstrumentName and
ShortInstrumentName the same and moved one of them for the
StaffGroup context, the other not.
You can move the ChoirStaff's shortInstrumentName almost wherever you want
by using an appropriate markup.
There's a mean little trick combining an empty markup " " with the actual
instrument markup so that a \translate will take effect: