I've been wondering when they'd get around to explaining Kakashi's Mangekyo as well. Damn, just how long have we been waiting for that story?
Anyway, Itachi said the trigger was to "kill your closest friend." But we can be confident that Kakashi didn't kill any of his friends, and Sasuke never actually killed Itachi--whatever poison/disease he had was the cause. Instead, we saw Sasuke's Mangekyo manifest when learning his brother's story: Itachi really was the brother Sasule loved and and admired (and tried to hate) all along.
My point being, what I think is the true trigger for Mangekyo is not specifically murder, but a psychological trauma, particularly the grief of losing your closest friend. If that's the case, Kakashi's activation of the Mangekyo was probably something uneventful, such as reflecting on his feelings when Obito (and presumably Rin as well) died.
There was never any mention of Senju DNA requsite for Izanagi use. What he'd said was something along the lines of Izanagi being the-ultimate-but-one-time-only Sharingan technique: complete revision of reality at the cost of eternal blindness.
It just so happened that Danzo had both Senju DNA and an arm full of Uchiha eyes, I suspect his dealings with Orochimaru yielded him the information about the contents of that stone tablet in the Uchiha ghetto (Kabuto revealed to Zombie Madara he'd learned the secrets of the tablet with Orochimaru thanks to rigorous investigation). Danzo probably had hoped to used the Senju DNA he'd infused in conjunction with one or more of his eyes, so that he might gain the Rinnegan.
... But now that you mention it, what are the odds that Tobi really is Madara? Perhaps a kage bunshin given individual life and personality by way of a last-resort Izanagi so that while Madara died, his grand plan would be carried on?
It's a stretch, I know.
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