Dec 28 2009 06:07 AM Marathoned this, then did brainstorming with /a/. [/a/ is pretty reliable when it comes to these things actually. Another instance is the Danzo sharingan hand.]
It all adds up if you analyze the subtle clues in the show. The Izanami - Izanagi myth is obvious. The Mitaka documents also say that Izanami's "womb" would bear fruit when the two meet, thus, Hagutsuchi.
The main plot element in this season are the Mitaka documents so all assumptions and deductions should be based off it. If the prophecy was fulfilled, then Hei didn't kill Yin, as far as that line of thought goes.
The most baww inducing thing was the line "The dream goes on" written on a piece of paper in the end which obviously refers to Suou.
How Yin became Izanami will be cleared up in the upcoming OVA[s].