Our courses are set up a little differently, but I can tell you something that may help. We don't conclude our courses until one month after the official end date. This is a part of the course creation. Students are automatically inactivated when their grades are posted. If a student needs continued access to the course after the official end date an incomplete is posted which doesn't inactivate the student. The student will continue to have access to the course for one month or until the course concludes unless we manually create an additional section, extend the date on that section, and add both the instructor and student to that section. Once the end date on the section passes the student will lose access.
Basically, access to the course is controlled by the date of the section of the course, it overrides the date of the course. Students must be enrolled in that section to have continued access.
It is possible to just adjust the date of the section to continue access, we choose to create a separate section.