Huh! on your view of the built in rating being per Author.
Have you looked at the structure of metadata.db? (with a SQLlite db browser)
The Rating table is tied (linked) to the BOOK ID, Not Authors.
As to rating source Priority.
If there is a calibre rating inside the book
eg
Code:
<meta name="calibre:rating" content="6.0"/>
That is used at the time of ADD Books.
If you
Download Metadata in the MDE, It depends on How you have that
Configured. That can have the top level set to accept any or the individual sources can be set (
cherry pick by source)
As to Priority. I thought that was what
Reading list PI did (I don't use that or even worry. I have a TBR list longer than my lifetime. YMMV )