1. I don't understand what you want to do with registration complete page. Could you please explain more details about it?
2. Sorry, we do not have a way to mark an event as it is full. However, when the event is full, there should be a message displayed to users, something like the event is full and we do not accept registration anymore.
3. Go to Events Booking -> Configuration, there is a config option called Show Number Registered Users, set it to Yes and number of registered users will be shown