Something that might help is to read about cohorts...this is like groups but at the site or category level. You can enroll cohorts in a series (path) of courses and require course A completion for course B completion (but not course B starting)--though this would be VERY complex with that many courses. I used to manage a site with (at the point I left the agency) more than 10000 courses and requiring prerequisite completion was practically impossible. Or you could enroll the cohort in course A and provide enrollment keys based on performance (or other criteria) for subsequent courses. Cohorts are awesome.
I *think* you can probably set up something in account creation that would add them to a cohort, which would start them on the curriculum path you wanted.
And as for completion/grading--you set grades for different items (like the AICC--which, in Moodle, you'll see a lot of SCORM references...it used to say SCORM/AICC, but it is my understanding the SCORM activity module will run AICC still--but I've never done AICC) and you tell the grade book how to look at grades and there is a grade report in each course. Course completion allows you to say what is required and then triggers 'completion' when those conditions are met--if you set grade requirements in activities, completing the activity without reaching the required score will not trigger completion. And you can set show if conditions on stuff--like if someone doesn't get a required score, it will display a message on the home screen, etc.