Regarding the category system - agree there seems to be duplication here. One possible use for categories is to hide pages that are being worked on - but I don't think they've ever been used for that on the doc site nor would I recommend we start. Therefore I would be fine with transferring/deleting the category system over to the tags, which I agree is easier and more apparent to users.
Regarding the tag page - you need some page to backlink to for the collection of tags to work (I think we forgot to say that in the instructions to creating a tag). So if we deleted tag, we could add the tag alias to the Documentation Status page. Then all tag pages to link to either tag or Documentation Status. That change would make a lot of sense - everything more centralized to the Documentation Status page - I like it!