Sorry I don't understand your prerequisites. In the first place, ELK works on its own. How does the plugin relate to your cluster?
If you want to remove and revert the once installed plugin, ask to the plugin developer will solve the problem sooner.
Anyway, can you access root elasticsearch url and log in to it? If you can and you know the password of user "elastic" (or any other user with appropriate role), you can use "change passwords API" and "create users API".
I found command line elasticsearch-reset-password tool for 8.0, but it is not supported in 7.15.
And I suppose this topic will also help you.
I hope some of them work for you.