figured bass extender lines

[Seng Liang]

Hi, I have a problem regarding figured bass extender lines in Lilypond version 
2.7.12 for Windows. Here is the file: 

\version "2.7.12" 
\relative { << 
\context Voice { 
\time 4/4 
\key c \major 
\clef bass
c,8 c c c d d e e f4 } 
\context FiguredBass \figuremode 
{  \set useBassFigureExtenders = ##t 
<6 4>2 <6 5>2 <5 3>4 } >> }

  Here, what I intend to do is to put the figured bass <6 4> for a duration of 
a half note and <6 5> for the same duration, and <5 3> with a quarter note 
duration. I would like extender lines to appear after <6 4> and <6 5> and run 
the duration of figured basses. However, 3 problems crop up.
First, the extender line from the figure "6" extends from the first beat until 
the 3rd beat, erasing the "6" from the <6 5> figure. How do I make the extender 
line stop before the second "6" and start again after it, stopping at the end 
of the bar? 
Secondly, how do I make 2 extender lines to begin after "6" and "4" for the 
first and second figured bass?
Lastly, if you look at the file, the order of the figured bass is from the top 
(<6 4> means 6 appears on top of 4). However, when I put <5 3> on bar 2, the 
order is reversed in the pdf file. Why is this?
Thank you.
Yours sincerely,
Seng Liang.