The Role "Designer" is a substitute name for a "Teacher" or a "Creator" renamed in the course?
If so, then I suggest you create the Role in the Site Administration > Users > Permissions > Define Roles, then the Manage Roles tab, and base the Designer Role on whatever existing Role you want, Teacher or Creator.
If the Role has been created this way, then go to the Allow Role Assignments and make sure that you have permission to allow that assignment.
To your other problem:
What can happen is a course gets too big and needs to be split up into sub courses. First, I suggest you might want to read these Moodle Docs on metacourses. What I have done is to take a backup of half of the course, then another backup of a second half of the course, and thirds if required, course materials only. No user data or materials. I have restored those to different, existing courses created specifically to act as shells for those backups making these parent courses. I then take a backup of Users and data, and anything else. I restore that backup to its own course and make the course a child course, and the other courses are metacourses. This makes backups of student enrolments and materials considerably simpler and easier for longer term storage.
This strategy has worked well for me, and would recommend it for anyone who is working with an evolving Moodle.