1) this information is included into FAQ section and available as at readme.txt and at http://shinephp.com/user-role-editor-wordpress-plugin/#faq
You can insert this line of code
to your wp-config.php in order to see 'Administrator' role in the roles list.
2) WordPress have separate storage for roles and capabilities for every site of the network. User Role Editor is just interface to manage WordPress internal roles & capabilities storage. That's why you should go to the site of the network in order to edit its roles and capabilities.
I just repeat WordPress way.
Yes, in theory it's possible to build the only role editor for all sites from one place. But do you edit some site settings from the Network Admin dashboard or go that site first and edit its settings there?
Although it could be a good idea for new feature: if you wish to have all roles and capabilities universal for the whole network, it will be useful to edit all of them from one place.
From other side, if you are the only admin who will edit roles, activate plugin just for main site. Edit roles there, save them with 'Apply to All Sites' option turned on. That's it. You updated all network from one place with current version of plugin.