extender lines in figured bass for Lilypond 2.7.12

[Seng Liang]

Hi, I have some problems regarding extender lines in figured basses for 
Lilypond version 2.7.12 for Windows 98. Here is the file content: 

\version "2.7.12"
 \relative c { << \context Voice { 
\clef bass
c8 c c c d d e e f4 }
\context FiguredBass 
\figuremode { 
\set useBassFigureExtenders = ##t  
<6 4>2 <6>2 <5 3>4 } >> 
}

Here, I would like to put <6 4> on the 1st beat with the extender lines until 
the end of the 2nd beat. Then <6> on the 3rd beat with the extender line until 
the end of the bar. The <5 3> is for the 2nd bar. 3 problems came up:

First, why did the extender line from "6" of <6 4> at the 1st beat continue 
until the start of the 3rd beat where <6> should have been, and now the "6" has 
disappeared. Why is this? How can I make the line stop before the 2nd "6" and 
then continue after it until the end of the bar?

Second, how do I make two extender lines of the same length lined up vertically 
to start from both "6" and "4" after the figure <6 4>?

Finally, why is the order of <5 3> in the pdf file reversed (3 on top of 5), at 
least it does on my computer system? Is this a bug?
Thank you.
Yours sincerely,
Seng Liang.