Short summary: User attempted to decrypt FileVault protection and got impatient; now can't get hard drive working at all. No file recovery needed (good backup, thank God), just need to get the hard drive to the point where I can actually work with it.
I have another system with a similar 1 TB drive, and used gpt to manually copy its partition scheme to the problem drive; it won't mount (from klanomoath's assistance to another user).
Also tried initializing the disk using Windows (NTFS); partially successful. It almost looks like FileVault somehow still has hold of the drive, even though everything, including the partition map, has been overwritten. Can't figure out how to just write enough of a format onto the drive so that Disk Utility has a place to start.
I just need to get this drive working, and don't care what data is destroyed. Can anyone tell me what I'm missing?