Hi Hayley,
no problem :) It's quite OK to set the restriction for the feedback, we also restrict access although more and more I'm asking myself why :)
However, I think that the option to track the completion of the Seminar activity as a trigger for sending the feedback reminder doesn't appear for you because the Seminar activity is not set in the Course completion as a condition.
To set seminar activity as a condition for course completion, while on the course page you can go to Course Completion and mark the seminar activity as a condition for course completion.
Step 1 (Image 1)
Step 2
After this is done, you can set the Seminar activity completion as trigger for sending the Feedback reminder (Image 3)
Hope this helps :)
Best,
Vlad