Yes that the point of using config_local.php, you just have to unzip the new package.
Thanks for the report.
For the metadata update : imagine you get an epub and when you import it on Calibre the title is wrong (it is Series : Book title) so you change it to Book title and add a Serie / Serie index and tweak the tags.
If update metadata is not enabled then the epub you'll download is the epub you imported on Calibre (so with the wrong title, no serie, wrong tags, ...)
If update metadata is enabled then the epub metadata (stored inside the epub file) of the epub you'll download will be exactly as in Calibre.
Hope it's clearer.