defining new context - alignement issue

[Frédéric Bron]

Hi!

I tried to play with the contexts and engravers but cannot reach
exactly what I would like. Please look at the example given at the
bottom of this message.
I would like the tempo marks, rehearsal marks, bar numbers and text
spanners to be written above and below each StaffGroup. I came up with
this example.
1. The issue is the vertical alignment of "Stringendo" and "Lebhaft".
Why are they not on the same line?
2. Also could someone explain the need of \consists
"Axis_group_engraver" (try to comment it and see that every thing goes
above the score..., three lines of marks).
3. Also if \override VerticalAxisGroup #'minimum-Y-extent = #'(1 . 3)
is commented, Stringendo is written on the first staff...

Thanks,

Frédéric

\version "2.12.2"

\layout {
        \context { \name "TextLine"
                \type "Engraver_group"
                \consists "Metronome_mark_engraver"
                \consists "Text_spanner_engraver"
                \consists "Mark_engraver"
                \consists "Bar_number_engraver"
                \consists "Axis_group_engraver"
                \override VerticalAxisGroup #'minimum-Y-extent = #'(1 . 3)
        }
        \context { \Score
                \remove "Mark_engraver"
                \remove "Metronome_mark_engraver"
                \remove "Bar_number_engraver"
                \accepts TextLine
        }
        \context { \Voice
                \remove "Text_spanner_engraver"
        }
}

Indications = {
        \tempo "Ziemlich langsam" 4 = 52
        s1*4 \mark \default s1*6
        \textSpannerUp \override TextLine.TextSpanner #'(bound-details
left text) = "Stringendo"
        s4\startTextSpan s2. s1*2 s2. s4\stopTextSpan
        \tempo "Lebhaft" 4 = 92 s1 \noBreak
        s1*4
}

music = \relative { \clef treble \repeat unfold 19 { c4 r c r } }

\score {
        \new Score <<
                \new TextLine { \Indications }
                \new StaffGroup <<
                        \new Staff { \new Voice { \music } }
                        \new Staff { \new Voice { \music } }
                        \new Staff { \new Voice { \music } }
                >>
                \new TextLine { \Indications }
                \new StaffGroup <<
                        \new Staff { \new Voice { \music } }
                        \new Staff { \new Voice { \music } }
                        \new Staff { \new Voice { \music } }
                >>
                \new TextLine { \Indications }
        >>
}