TBH I think you will get more complete answers from an Audio - Audi forum... Have you looked on AudiWorld's audio forum? There are some no-it-alls on there.
Not wishing to send you from here, but it looks like you've waited for long enough hey.
Yes the L is for LWB, to be honest the rear leg room in the SWB is great, but for taking clients in the back it is worth it. It does have a downside thoygh for the sporty driver, it affects the ride and cornering, but not such a deal for the 3.0TDI.
VAG-Com is a diagnostic tool (software and comms lead) to interogate fault codes and such like. Unfortunately with most modern vehicles it is not easy to do without such a tool. VAG-Com would tell you what sensors are giving out of the norm readings and will display logged error codes.
I'm now on my second A8 :o.k:
My first was a D2 shape 4.2Q (2000 reg) I bought that at 2ysr old with 92k on the clock, I took a bit of a risk I know, but at the price it was I couldn't resist. As it happened it wa the second best car I have ever owned. As for fuel consumption... guess it...