Hmm, actually if I think about it, I believe the plugin overrides the setting in Output options. Just use the "authors" field and not the "author_sort" field in your plugboard, and make sure the calibre "authors" field does not have any commas.
You can also inspect the metadata of the book after it is sent to the Kindle -- easiest way is via calibre's command-line backend:
Code:
ebook-meta path/to/book.azw3
The author_sort value (in brackets) should not have any commas.