chaley is the expert on these matters, but I'll see if I can help.
The thumbnail issue and proposed workaround is discussed in
this short thread.
As for your author_sort problem... I've just sent 50 books to my PRS650. After disconnecting, the author_sorts appeared correctly. I then reconnected and disconnected again. The author_sorts are still intact.
I don't have the "Use Author Sort as Author" option enabled in the Sony device plugin, I have moved it into my Metadata Plugboard. Do you have one of these set up for your Sony?
How are you viewing your bookshelf when you see author instead of author_sort? In Collections ? See all by Author?
Have you double-checked the actual values of author and author_sort in calibre edit metadata for a badly behaving book?