Try XnView - very comprehensive (free and you can use it as portable app)
Uses the IPTC tags - and you can batch tag etc.
XnView can do everything you ask AFAIK, excluding the ability to handle such huge number of pictures with ease, and mostly because I don't have so many to test
I tried XnView. It does look like a nice viewer/browser but, as an organizer, I couldn't get the hang of it.
Maybe I am missing something...
Exactly unlike you, I am using FastStone and happy about it. But what I need is more editing that organizing/tagging.
Ditto for FastStone.
A very interesting approach to tagging images is employed by Viewer2. I really like it.
The concept looks interesting but I don't see how it would scale to a couple of hundred thousands images.
I decided against installing it for now. If I am wrong, feel free to point it out.