First things, I find your theory reasonable and similar to what i myself suppose. As an exercise lets turn "awakening mangekyo" into an abstraction. The abstraction, and in duality; theme, is "trading love for power". We can all agree that the uchiha descended from the older brother. We can all agree that the older brother preferred power and force whereas the younger brother was aligned with the path of love. So, killing your best friend to awaken mangekyo is a manifestation and reminder of the "curse" of those descending from the older brother.
So, killing your best friend, IMO, is not a requirement in any strict sense. What transpires within an uchiha, depending on circumstances pertinent to his life, is a change of state in any given bond or acknowledged love. That then, forgive me for the word i am about to use, that then begins a "mystical", as well as physical and spiritual transformation of the severed bond, lost love, what have you, into energies constituting the mangekyo.
What we have now with the rule of killing your best friend is a LEARNED BEHAVIOR, the result of path the older brother embraced. Darkness.
Mangekyo is not only trading love for power but even moreso "blind with power". Mangekyo causes loss of vision for reasons well beyond ninja medical science. It's mystical, and tied also to learned behavior. Mangekyo, in the manga, has been mostly shown to be cast fro
Shinobi who walk totally or partially down the road if figurative darkness. The power of the eyes being used for purposes that cause one to walk into darkness will then guarantee the curse, you will go blind because you are blind with power.
Since kakashi has the will of fire, walks away from darkness, uses mangekyo for virtuous reasons, his mangekyo might behave subtly different in regards to the typical penalty of blindness. It may be doing something else entirely that perhaps may turn out to be a boon, unlike the doom of an uchiha who uses mangekyo in the darkness.
Itachi realized all this too late. He tried to tell kabuto the importance relying on your comrades. It all boiled down to the simple lesson personified in the two brothers, itachi realized the correct answer is the way of the younger brother.
I think the sharingan is tied to morality, justice, and character. It's power will depend entirely on the path of the user. It can go many ways after this as to what is or isn't possible.
You catch my drift im sure.
Again, kakashi has not yet suffered deterioration of his vision for the simple and irrefutable reason that his kamui attempt on the mazou showed his vision to be healthy.