Bar lines

[Noeck]

Hi,

I tried the (relatively) new bar line interface:
\version "2.17.12"
\defineBarLine "|:" #'(":||" "" "|:")   % a)
\defineBarLine ":||" #'(":.." "" " ||") % b)

\new StaffGroup <<
  \new Staff { a1 \bar "|:" a \bar "|:" }
  \new Staff { f'1 f' }
>>

I have four questions:

1) When using definition in line a), LilyPond complains about a missing
span bar line style. Should I add line b) to define it?

2) Is the space in " ||" right in line  b) to align the span bar lines
properly?

3) Having a ":" in a span bar line style (like in a)), it is printed
above the staff instead of in between the staves (where I would have
expected it). Am I wrong to define it this way? Or is this a bug?

4) Would it make sens to have a fourth bar line situation (besides in a
line, end of line, start of line): The final bar line at the very end of
a staff?

Cheers,
Joram