Moodle will essentially treat MNET users are local moodle. With regards to the enrolment options, I haven't personally tried MNET enrolment with a key, but I don't see any reason why it wouldn't work. I also found this http://docs.moodle.org/23/en/MNet_remote_enrolments . I am sure you must have see this, but I figured I would share the link anyway.
They will need to login to Site B and then click on the block link. However, there are ways in which you can convert MNET users to local users. You could schedule a cron to convert new MNET users to local users. This would allow them to login directly from Moodle A.
Lastly, before you seriously consider MNET, please note that MNET support might be dropped in the future versions of Moodle. https://tracker.moodle.org/browse/MDL-40905