This plugin will immediately invalidate your auth cookies when you manually log out.
No. This plugin will only invalidate cookies that were created after activating the plugin.
Most likely, no. This plugin is only meant to be used with the standard auth cookies that WordPress uses.
This plugin overrides the core
wp_validate_auth_cookie function. This means that you can't enable this plugin and another that also overrides the same function.
This is a non-comprehensive list of other plugins that also override this function (and should not be used at the same time as this plugin):
It's also possible that if another plugin is overriding a related function (e.g.
wp_generate_auth_cookie) that this plugin will not work correctly.
Once activated, if this plugin is NOT overriding the function, then a message will be shown to admin users towards the top of every admin page.
Simply rename or delete the plugin so WordPress can't find it. This step requires that you have access to the filesystem where WordPress is installed (via FTP, SFTP, etc).
There's a small chance that this might happen. It most likely happens because of an incompatibility with another plugin that also overrides one of the core