Final Cut Pro X 12: Cannot delete empty event

Except in very specific circumstances and with great care, you should never mess with the innards of a library using Finder.

Final Cut Pro relies on sqlite databases to manage the contents of the libraries. By deleting files manually from an event, the database does not match the file contents.

At this point, I would try making a new library, copying the events to that new library - in Final Cut Pro - and if that goes well, stop using the old library completely.

The way to handle the media depends on where it is stored now.

For such a big library, I would have kept the media in place when importing, which would make the copy above very quick and painless. If, however, the media is kept inside the library (which makes the library huge, and more susceptible to corruption), you then need to copy the media, together with the events (which, fortunately, in APFS, does not use extra space, and is quite fast)

Open the library from a backup. By default they are stored inside ~/Movies/Final Cut Backups/

Is the library on an external drive? If so, how is it formatted? Drives that hold libraries need to be formatted as APFS or HFS+.

Backups of data are vital, always have been, always will be. If you can't afford a backup drive, then you live with what you have. I have very large Libraries and none have ever corrupted. I also will reiterate that you should NEVER go inside a Library package and mess with things, because you end up with corrupted stuff, period. If you manually delete media from inside a Library, you're asking for trouble. You corrupted that Event by doing that.