Centering tremoli under noteheads and suppressing lines through/above/below off-the-staff notes

[Mike Solomon]

Hey all,
    I'm working on a piece with a lot of birdsong & I'm using notation that
looks more or less like the snippet below.  Before I work out my own hacks,
I was wondering if any of you had quick fixes for the following two things:

1)  Is there a way to suppress the lines through / above / below over and
under the staff notes?  In this example, you'll see that the high A has a
line through it, and I'd like to avoid that if at all possible.
2)  The tremolo is typeset where it would show up were there a stem - is
there an easy way to center it under the note?

Thanks!
~Mike

\version "2.13.30"


lb = \relative c''' {
  \override Staff . StaffSymbol #'transparent = ##t
  \clef "treble^8"
  \override Voice . Stem #'stencil = ##f
  \override Voice . Beam #'stencil = ##f
  s16 \once \override Voice . NoteHead #'font-size = #2 \bendAfter #2
a16\p\< \
  a16-. a16-. \once \override Voice . NoteHead #'font-size = #2 \bendAfter
#4 b
  \once \override Voice . NoteHead #'font-size = #2 a4:32^\markup { \italic
"je
  \once \override Voice . NoteHead #'font-size = #-2 \parenthesize c,16 s8.
  \revert Voice . Stem #'stencil
  \revert Voice . Beam #'stencil
  \revert Staff . StaffSymbol #'transparent
}

\score { \new Staff { \lb } }