Hello,
2) Yes, I’m afraid the Events module is designed to use an existing user account for cross-referencing the user info.
5) I think email has to be unique in the DNN core, but I may be wrong on this – it is possible a configuration value in the web.config.
7) No, I actually wrote a module called the Smart-Thinker Scheduler but never finished it – it was capable of sending event reminders and notifications to the Event host with summary info. Registration cannot be closed on a date – this would require customization.
8) I can’t think off-hand of one, but maybe something like XMOD – IMHO the moment you start dealing with custom business logic you need a custom module. For example, on PokerDIY, the poker leagues (the core business function of the site) is a huge custom module as no other module could be configured to do what I wanted to do, or I would have used it (it took a LONG time to write).
Out of interest, I once specced out a system to allow you to RSVP without an account – you get an invite with a unique key and you can click on the event and RSVP and then decide if you want to join the site later. It was a fairly big chunk of work so I never went ahead with it.