1. The export is for all sessions. You can print a sign in sheet or copy it to a spreadsheet in less time than it takes to export.
2. Actually, it is checking overbooking that puts people on a waitlist. If you don't check it, the session is hidden from further views. If you turn on the third party e-mail for the activity and list the instructor's e-mail, it will send a message for every registration/booking and cancellation. That would keep the instructor apprised of what's up. I'm not sure the administrator would want to know/care. I admin a site of more than 4000 active accounts and several hundred active F2F activities. I don't want to know. That said...can it be changed, sure. If you've got someone who can modify code and fiddle with stuff (read: hack the code).
3. Capacity is just a field. When you (or the instructor) creates the session, that's when you identify the capacity.
4. Probably to both questions, as I said in 2. But keep in mind any modifications to the code will mean updates will become difficult.
5. See above. This would probably be the easiest code hack and least potentially destructive.
A question for you would be this: How willing are you to break it? Because customizing can result in unintended consequences.