I’d prefer new topics for new issues in the future 
You need to use the --local flag for mmctl everytime you run it in order to use the local socket based authentication so you do not need to login with a username and password to Mattermost.
For the deletion of users you might stumble upon another hurdle. I think by default, deletion of users via the API is forbidden, but you can change that:
# mmctl --local config get ServiceSettings.EnableAPIUserDeletion
false
Use the set command to change the value to true if it returns false for you.