Changing context plugins

[David Sumbler]

This question arises out of a problem I had in the piece I am currently
setting.  I have certain marks I wish to appear above a bar-line.  I
want one of them to appear only in the lower stave.  In this particular
instance I can get the result I want thus:

%%%%%%%%%%
\version "2.19.48"

\language "english"

firstmark  = \mark \markup { \whiteout "First" }
secondmark = \mark \markup { \whiteout "Second" }

\score {
  \new StaffGroup {
    <<\new Staff { R1 \firstmark R R }
      \new Staff { R1 R \secondmark  R }
    >> }
  \layout {
    \context { \Score
               \remove Mark_engraver }
    \context { \Staff
               \consists Mark_engraver }
  }
}
%%%%%%%%%%

But two questions have occurred to me arising out of this.

1) Plug-ins for most contexts can be added or removed either in a
\layout block, as above, or when creating the context, e.g.

\new \Staff \with { consists Mark_engraver }

But this second method can't, so far as I can see, be used with a Score
context, because '\new \Score' is not allowed.  Can Score plug-ins only
be added or removed within a \layout block?

2) More importantly, is it possible to change these settings at an
arbitrary point in the music, i.e. after the context has been created?
 I can't find any way to do that, although with my limited
understanding of the internal workings of Lilypond, there doesn't seem
to be any reason why it should not be possible.  (This is what I
started off attempting to do in my piece, without success.  I then
realized that in this particular case I could work around the problem,
as shown above, but in another case one might need to change things
part of the way through a piece.)

David