In the event that an MP3 file may be poorly tagged or have no tags, and is listed among 20,000 other songs, how do I locate the song in the "Local Files" list?
The local files seems to actually be added in order to the local files list as they are found, meaning that if you move/copy only one directory with a few tracks into one of your directories that are scanned for local files, they will show up together (but probably unordered as they are found on the filesystem) at the bottom of the list as long as you have no column ordering. I think that view defaults to order on song title, so click the song column title a few times until the sort order is removed.
There are probably lots of way for us to improve the local files integration. One thing that was never implemented in the new client, but available in the old was to be able to drag files directly to playlists without having to add a directory to scan recursively for all files and then find those tracks and then drag them to playlists.
Another way would be to expose filenames or ingest date/time and/or index newly found files and directories in alphanumerical order.