I deleted all books and refilled the Kindle and the prob was gone. Hickup while filling it in the first place?
Anyway the sorting is all right now. And, for my taste, even better: eschwartz somehow inspired me to actually play with the author_sort field in the plugboard and I tested a while (never changed that field before in the plugboard).
Turns out: it's possible to trick the PW2 into NOT switching "lastname, firstname" to "firstname lastname" but show the content of author_sort instead in the books list and inside collections

I love it!
Also I added the series and series index to the author now.
In the book list (filter "books", "collections" or "all items") I have the following order now:
<book title>
<author_sort B>,
<book title>
<author_sort B>, - <series A title> - <001>
<book title>
<author_sort B>, - <series A title> - <002>
<book title>
<author_sort B>, - <series A title> - <003>
<book title>
<author_sort C>, - <series B title> - <001>
<book title>
<author_sort C>, - <series B title> - <002>
<book title>
<author_sort C>,
<book title>
<author_sort C>, - <series C title> - <001>
<book title>
<author_sort C>, - <series C title> - <002>
<book title>
<author_sort C>, - <series C title> - <003>
<book title>
<author_sort C>, - <series C title> - <004>
...
...
...
The little comma added to the author_sort field via plugboard did the trick. That it also shows everywhere next to the author is not an issue for me
I also have one collection named <''All book series> now that sits on top of all other collections. Inside are all my series ordered by author_sort, then series, then index.
Now I have all my Sony reading and handling comfort back plus all the goodies the PW brings