Do inactive accounts remain here forever?  I had "friends" who had not been on this site in 5 years.

Also, is there a way for members to cancel/delete their own accounts?

I don't see any such option in settings, though, I think I have read before that someone could write Tom and ask to be zdisabled?

As far as I understood they won't/cant be deleted due to the needed legacy data for the rankings to work, but as berickf said I think you can ask Tom to switch it too a zdisabled number.

Though I'm not sure why you would do that in case you ever decide to return