I am studying the Quran currently, and Islam in general, though admittedly at a rather introductory level and more in relation to the modern world as opposed to ancient Islam. So I fully acknowledge that I am likely out of my depth in regard to many of these things.
However, from what I've studied it seems to me that for something such as the unification of Islam and Christianity, either Islam would not be able to be Islam or Christianity would not be able to be Christianity. What I feel more likely is for some Prophet or Messiah to attempt to unify the two religions and as a result create yet another Abrahamic religion that neither Muslims nor Christians accept as valid.
In any case, it's a bit difficult for me to follow your reasoning as I'm unfamiliar with some of the terms you listed, namely Istibdal, Istihal and Shirk. I'm also unfamiliar with whom Nejashi is. If you could direct me to where I could go to better understand what these are it would be much appreciated; I've still got a lot to learn and I enjoy doing so.
Finally, I'm curious as to what you mean when you say that Christians and Jews can be considered 'in the fold of Islam' but in deviant forms. Do you mean they can be considered Muslims, or do they still have to pay jizya under an Islamic state?
I forgot to mention who Nejashi was, forgive me lol. Nejashi was the king of Axum during the time of Muhammad who was a Christian who gave the Muhajaroon (Emigrants) asylum from the Quryash, and ruled in favor of the Muslims in court between them and the Quryash sent to bring them back. Muhammad called him just and a pious ruler before Islam or the knowledge of the Shahada reached him, thus he was a Mu'min (believer) without recognizing Muhammad because he followed the Tawheed and did not commit any of the 10 forms of Kufr and ruled by what was law in the Injil (he ruled by Old Testament laws, and thus the laws of Allah).