RSVP to the whole serie is less used in my experience with events than being able to subscribe to a precise event later in the serie; both options are important though.
Yes, however I don’t see on my installation any workflow that does that.
It’s maybe in itself another feature request and interesting beyond the scope of events topic and I could/should create another topic for this.
We’d need an automatic archiving action triggered on certain conditions; here it would be “event date or end date if present” is in the past or topic is closed since x days, with maybe a chooseable delay before the action occurs so that event topic can still remain visible for people to comment after the event for a few days.