As per:
https://community.nintex.com/t5/How-To-s/Remove-Unused-Nintex-Workflow-Content-Database/ta-p/87264

"nwadmin -o DetachDatabase -Servername SQL_ALIAS -databaseName NintexWorkflowDB_Custom

Does not remove a database if the database no longer exists in SQL.

You receive the error:

Command line execution error: Cannot open database "NintexWorkflowDB_Custom" requested by the login. The login failed.
Login failed for user 'CONTOSOsvc_farmAdmin'.

There is no -force option on the command and it doesn't seem to proceed with removing the rows from the configuration database.  I'm less inclined to actually want to go in an edit rows in the DB just in-case it leaves something wonky.

Any solutions for this problem?  Could NintexDev maybe have the command complete the operation properly if the database doesn't exist?