Thank you Davo - you're right, I've changed those permissions so much I wasn't paying attention what I pasted back in here. Going to go back and fix that now.
I've concluded that what I'm trying to do is a little outside of the standard Moodle workflow. While possible, it means hacking around the fact that from what I see, Moodle is a inclusive system and separating one set of users from another, based on a very specific set of permissions is quite hard. Its possible by perhaps introducing flags into accounts and things, but its more work than its worth at this point.
In this light, I've decided to make our misc users non-existent, in that, the action of filling out the assessment is done without creating an accounted, locked behind a code. Which now thinking about it probably makes more sense anyway. This way, everyone else in the system is considered someone that should be part of the program.
Thanks!