It doesn’t work the way you think it works. I’m going to answer your questions one by one.
Login redirect doesn’t automatically add roles to it. If you have records exists in that screen, then someone must have added it.
Usually when you don’t want a user to have access to wp-admin, you don’t want them to see the toolbar. So when the user tries to access wp-admin, you want them to redirected to a different URL. Hence this functionality is part of that.
When you don’t have a role configured within login redirect, the role should work as normal. The toolbar or the wp-admin won’t be disabled. If you see them as disabled, make sure its not coming from a secondary role.
If you want to use this functionality in future for one role and you don’t want to add other roles, other roles will work fine. The toolbar or the wp-admin won’t be disabled for those roles just because you didn’t add those roles in login redirect. If that was the case, then every role should have toolbar and wp-admin disabled when nothing is configured.