Dear Kaj,
you are brave to really dig through all this! That's a good thing. I
think I can help you here, too:
It is correct (and it works like expected) if you do
\override StaffGroup.StaffGrouper.staff-staff-spacing.padding = 30
like I wrote. But only (!) if your grouping context is a StaffGroup. In
case you have a PianoStaff or a ChoirStaff or anything else, you need to
override the property of that context. I.e.
\override PianoStaff.StaffGrouper.staff-staff-spacing.padding = 30
and the like.
There is a context hierarchy in LilyPond: A lower level context (e.g.
Voice) can be inside a staff, which can be inside a staff group, which
can be inside a score. Of course more complex nesting is possible.
The important thing for you is: The inner contexts inherit (take the
overrides) from the outer contexts. In your case, if you override the
StaffGrouper of the Score, that overrides the spacing for any
StaffGrouper in the Score. If you override the spacing for the
StaffGroup (and similar), only this kind of group is affected and if you
do so in the \with statement, only this particular context is affected.
Here is an example showing some possible overrides:
\version "2.18.2"
\layout {
% no effect because there is no StaffGroup
% only PianoStaff and ChoirStaff
\override StaffGroup.StaffGrouper.staff-staff-spacing.padding = 60
% this works for the piano staff (upper group)
\override PianoStaff.StaffGrouper.staff-staff-spacing.padding = 15
% this works for all groups but the PianoStaff has more specific
% settings which are used instead
\override Score.StaffGrouper.staff-staff-spacing.padding = -2
\override Score.StaffGrouper.staff-staff-spacing.basic-distance = 0
}
<<
\new PianoStaff <<
\new Staff { a }
\new Staff { b }
>>
\new ChoirStaff <<
\new Staff { c }
\new Staff { d }
>>
\new PianoStaff \with {
% this override only affects this second piano staff
% overriding the Score and PianoStaff overrides from
% the layout block above
\override StaffGrouper.staff-staff-spacing.padding = 4
} <<
\new Staff { e }
\new Staff { f }
>>
>>
Don't hesitate to ask again if things are still unclear.
Cheers,
Joram