I'd use the auto-generated (if enabled) recently added Playlists or an AutoPlaylist sorted on Added. This way you can quickly go through the newly added files identifying any that need fixing.

D:\A\Z\Zen Alpha\Mixes EP\Dance (Hey).flac

D:\A\Z\Zen Alpha\Mixes EP\Dance - Hey.flac

D:\A\Z\Zen Alpha\Mixes EP\(It's A) Dance - Hey - Ho (He).flac

Obviously, my example could be easily accomplished using the Advanced Search function; I only proposed it as a simple example. I'm more interested in the general case of searching the Library database and finding all Artists whose name is a single letter (and/or a single word) then adding that entry to a custom Playlist. I could then manually scan the playlist to repair malformed entries/tags.

MediaMonkey occasionally gets it wrong when it automatically identifies an artist. If you have a file name which looks like:
[code]
D:\A\Z\Zen Alpha\Mixes EP\Dance (Hey).flac
[/code]
it will identify the artist as: Hey. Another file name might look like:
[code]
D:\A\Z\Zen Alpha\Mixes EP\Dance - Hey.flac
[/code]
it will identify the artist as: Hey. This file name might also cause problems:
[code]
D:\A\Z\Zen Alpha\Mixes EP\(It's A) Dance - Hey - Ho (He).flac
[/code]
in this case it might inappropriately add different entries to both the Artist & Album Artist fields.

Why making a script for this, as it's so easy to achieve it manually with the Advanced Search functionality?

I would like to create a script which searches the entire library looking for entries (e.g. Artist is a single word/letter) which match some criteria and add them to a (acceptably: already existing) Playlist for manual examination. As a simple example, search the entire Library for every entry whose Artist is exactly "A" and add that entry to an existing playlist "Playlists:Alpha".