There are so many different ways you can enrol, I think you need to look at all the options.
1. External database/LDAP: you could set up an external database or LDAP server and use groups for enrollment/unenrollment.
2. Upload courses and/or users. You can use csv files to create user enrollments and create courses.
3. A lot enrollment methods have expiration dates which you can set at the course level. This will take care of your time limits.
4. You can pull grades from the database without having to reenroll the students. Look at the configurable reports plugin.
Sometimes we get stuck looking at trying to do something one way where really we would be better to think a little differently!