or perhaps the way the Mangekyou Sharingan manifests in people depends on the way they first obtain it? (ie. killing their best friend, or whatever Kakashi has discovered)

still, that's saying that Kakashi actually has the Mangekyou Sharingan, and not some other kind of upgrade to it. Then again, the conversation with him and Itachi's clone...

Kakashi's Sharingan has always been the biggest "where the fuck did that come from?" question mark of the series for me. And now it's even more so.