\version "2.12.2"

fb = \figuremode {
\bassFigureExtendersOn
  % Question 6: With extenders, subsequent new numbers always appear below existing numbers, which is wrong:
  <_+>2 <9 7 _+>
}

music = \relative c' {
  c2 c |
}

\new Score {
  \new Staff <<
    \new Voice = "FiguredBass" { \fb }
    \new Voice = "Music" { \clef "bass" \music }
  >>
}