If you disable Membership from General > Settings then user can’t register on your website with PR form also. Go to PR menu > Settings > Overrides page. Make sure WP default login page is disable. See screenshot: http://prntscr.com/nikc0y
Also, make sure that Admin approval is selected in PR menu > Settings > Security page.
The settings you advice are the settings I did have and gave me the problems.
With these settings I get the new users registrations bypassing the PR form & get immediate approval.
Actually after changing the general setting to “anyone can register” I got a new user within 5 min.
I also enabled the invitation code option. The registration just ignored this option.
It it possible to use a script to get an account without using the PR form?