Success in the end. I deleted all the instances of Kontakt 6 the finder told me about. Yet, weirdly, when I started Live and opened the vst I had previously saved to my favorites list (one of my color collections in Live), it would still start up. I removed the favorite and reinstalled Kontakt 6 a second time. Now it works.
I know this looks like I just forgot a particular instance of Kontakt, but truthfully I thought I’d caught ’em all. Very strange of Live to be able to launch a favorite I’d supposedly deleted. Again, I suppose I must have overlooked something. But just in case, try deleting Kontakt from your favorites too.
As far as I’m aware the Collections in Live are just shortcuts to save you having to shuffle about in the menus so much, but I don’t know why the behaviour I saw was occurring. NI themselves are pretty weird anyways so who can say? Good luck everyone.