If I have an Achievement rule (e.g. Expert) set up for some actions (e.g. ran 100 reports), is there a way to tell who is the nth person to earn this achievement?

I don't think that is currently supported. This information would only be present in the game activity log and I know using that for this purpose won't be easy.

I think this would be a nice feature to add @khs, maybe let us try to have it in the near future ?