Kyuubi no Shinigami wrote...
Or Tobi was Madara,
I am going to assume you mean Obito instead of tobi here. And I don't think Obtio could be madara unless madara somehow found a way to make himself younger. And if he was Obito then why did he give Kakashi his Sharingan, What does he stand to gain from that.
Kyuubi no Shinigami wrote...
could explain why Kakashi was able to somehow gain the Mangekyou.
I don't think so reason behind this. Is that if it was Madara's eye that Kakashi has, wouldn't the Mangekyo's look the same. There has been nothing saying that they would look different (or the same for that matter) if one eye was in a different person then the other.
And here is images showing Kakashi's Mangekyo and Madara's before and after he took his brother's eyes
Kakashi's
Madara's brfore
Madara's after