1. Well sign up link in sleek header type near the login which you are using wont be possible as of now .But you can translate the text "login" in the header to "login/Sign up" using this tip :
https://wplms.io/support/knowledge-base/translate-using-loco-translate-plugin/
2. There is a setting in wplms which does not publish the instructor courses without approval .
Please set "
Admin Approval for Course " in wp-admin -> wplms -> course manager to "Yes , Require Approval".
3. Well then you can use other security plugins for wordpress .