Hi Bianca,
I'll start by answering your second question : I would create a course and the QR code directs the participants to this module. In this case, the registration to the course have to be setted. On this subject, here is what I practice: the participant is included in a cohort and only the members of the cohort can register via the qr code.
On the subject of validation at a seminar via a qr code: here is how we could proceed: creation of a single activity course with inside a feedback type activity. Participants use the qr code to join this feedback.
Hope this helps you or contributes to your thinking. In any case, I would be very happy to know other methods about how to use qr codes with Totara.