That's a lot of changes in one post... I'll check that later.

For the VW T4 and up, indeed since some time these should be changed, I'm surprised we never did it in the end. I could change for the T1 to T3 to use the mk field also then.

About the VW Golf, Wikipedia use 5G too.
For the Lupo they mention 6E rather than 6X, but anyway indeed 3L is not right I guess.