Someone could change the topic they offer each week

If the same topic comes up twice, would it have the same spelling and case?

it's quite likely that visitors will take part in sessions

That answers that right there. :)

Email addresses are stored in the People database as an ID, but can be changed. The auto incremented internal ID is the "authoritative" identifier in the system.

One reason to sue an email address or the like is so the FKs make sense. No reason to hide information behind a number. As people may have the same name, ids are usually used, but a company email address or the like can help it be more meaningful.

Regardless, i posted some tables and a view below that might help. If you like the approach but want a small change or two, please just say so!