Use Administrator Password

In "Settings -> Use Administrator Password". Note that if the plugin is active, then you can log in by entering any valid username together with the password of any user with administrator privileges. The settings are additional settings, to allow further possibilities as well as this basic one.

Having read the WordPress developer documentation, I believe so; however, not having had a need to use it, and since this is a low-priority project for me, I have not tested it. Therefore, you should do your own testing. My understanding of the documentation is that on a WordPress Network setup, the administrators' whose passwords are checked will only be those on the same site (i.e. not network-wide); however, I repeat, I have not made time to test it. (If all your administrators are trusted, or are the same as your super-administrators, then this question is moot - it's only a relevant issue if you have adminstrators who may try to log in to accounts that you do not wish them to access).

Please either send a patch, or make a suitable donation on my donation page, and I will be glad to help. Otherwise, this plugin does all I wanted it to do and I've not got time to develop it further.

That's nothing to do with this plugin. This plugin gives you an extra way to validate a login (by knowing an administrator's password), but does nothing else to remove or lock-down any other authentication settings which you have.