If you were actually familiar with the Qur'an and history, you would not need to refer to a verse 18 verses later in order to understand the context. The verse is referring to a specific incident in history, as is plain. Your taking this verse and making it an alleged general proclamation of how Muslims should behave towards those who do not believe as we do is fanciful and typical of ideologues among Atheist and Islamophobes who jump through hoops in order to construct arguments in support of their arrogant and self-righteous attitude towards Islam, the Scripture and religion in general. That the verse coming 18 verses later being a complaint of yours is incredibly lazy and evidence for Hyd and completely undermines your criticism of Ustadh Nouman Ali Khan. It is furthermore evidence of the contention of people like Charle Le Gai Eaton that the fundamental problem with Europeans scholars (and Western civilization in general) is that when they read the Qur'an, they demand it conform to their mode of literature, instead of relating to it how it is. Education can change minds indeed and your extremely odd claim that 8:12 is ordering the mutilation of bodies begs that you continue yours. If your "scientific" mind has left you so unable to competently read "non-scientific" literature, then you stand as a proof against yourself.
In regard to the so-called Caliph, we have already answered you. If our answer does not fit into your preconceived argument, that is your problem, not ours. I take it that you clearly either did not read the link I supplied or you simply do not care to let things like facts get in the way of your opinions. In answer to your question: among all the things that could be said, the simplest is there is a concept called "bayah" in Islamic law and it is the un-coerced, willful agreement of the Muslim polity that they consent to a particular individual being their leader. So excluding everything else that you have wantonly ignored, no one can unilaterally declare themselves caliph and then proceed to "cut off the heads and fingertips of those who are fighting them". ISIS's declaration is essentially a dismissal of all rival groups and an empty claim that they alone are the legitimate political entity among the Muslim world. If you really understood current politics and did not simply limit yourself into periodically provoking us into shallow arguments, you would have understood clearly understood that the silence of this forum was an answer to the call. Even the Taliban have enough common sense to see ISIS's declaration for what it is. Why don't you?