Hi Cathy,
This is expected behaviour as far as the SDE plugin goes, if there are no spaces on the seminar then the plugin will not enroll the learner. I think the issue here is your customisation of the UI.
You could include guest access in any courses which are using the SDE plugin - this will let the user see the content without enrolling them, I'm not sure how this would work with your customisation but its definitely worth a try.
Hope this is useful,
cheers,
George.