re AO3 Tag count being larger than work count - I can think of two main reasons. (1) If the tag count includes bookmarks, and (2) Tags removed from works, and those tags haven't been cleared from the system yet. Neither should create tag counts "much" higher than the work count, though. It's also possible that (3) some works are double-tagged - like someone tagging both Snape/Harry and Harry/Snape because they don't realize they'll search the same way. (I haven't checked the data.)
Oh, thanks for the ideas!! It hadn’t occurred to me that bookmark uses might be included (or I missed that fact, if it was documented). I was seeing roughly 2x the expected count for some of the tags I looked at (but not all). Bookmarks and double-tagging might well account for a bunch of that (and deleted uses for some of it).