One of the reasons I asked (and Kovid did) for the Single instance (reusable) viewer option was for checking inside the book, to allow me to fix each books metadata, my way
(I enter the actual paper version> Imprint in Publisher)
In the Viewer: Preferences: Miscellaneous: General: <near the bottom
With this ticked, you just open a book, look, fix the metadata, open the next book. No need to close the viewer, it will be reused
There is no substitute for TLC when striving for clean metadata
(you sure won't get it from the download sources
)