re: Need advice on handling workplace violence/threat/harassment.

Find an employment lawyer.

The evidence you have is substantial enough that most lawyers will take this on. Any new recordings would be beneficial, but they’ll be on the lookout for that. Proceed carefully.

Also, you will not be working here much longer. Quietly start looking for another job.

There’s a chance that you will be receiving a substantial severance offer. Don’t initially accept it. It’s probably 1/4 of what you’ll get. A good lawyer will squeeze them pretty good.