Hi Michael,
I'm in your country, not mine, so don't have notes to look at on this. My problem w/ 2 student roles is that you'll then have to determine when they are Type 1 and when they are Type 2, which is not an easy technical solution.
We have experimented with what happens when you unenroll & reenroll a student. One way, when the student is re-enrolled they are a new person. Another way, they return to the classroom and all their stuff (quizzes, etc) still exists and is theirs. I believe that the second method was reuploading them into the course in a csv with their username and id number identical to the previous enrollment.
If you can get that right, then you don't have to worry about lecturers unenrolling your students. You simply
upload a daily .csv from your SIS (student records). Anybody who was unenrolled and shouldn't have been is reinstated.
BTW, we've never had a teacher unenroll a student, intentionally or otherwise, and we've been running courses in Moodle for 2 years now. Large number of courses. Teachers with essentially no CMS training. Still, not this trouble.
So, then you can just scare the heck out of your lecturers about unenrolling people and teach them how to do the Assign role action in course administration. This allows them to add an "FA 2" student as long as they want.
An alternative method would be to have a human add the "FA 2" students to the daily .csv. Use the .csv to enroll from student records and to enroll/unenroll the FA 2 students.