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?
By deviant I mean they would become like the Zayydi (Shi'i) and would still be Muslim in (I'm speaking from the point of view of Ahl Sunnah wa l'Jama'ah better known as Sunni Islam) regards to Fiqh but deviant in some views. The Shi'i in general are considered by all as part of Islam but are deviant in many ways, there are many rulings on this as well. The Christians and Jews would become like that, if they followed the terms and didn't commit the terms I am about to explain (which Zayydi Shi'i usually agree on).
Tawheed ul-Uluhiyyah is that Allah alone should be worshiped alone and none other than him can be singled out (including Muhammad or any of the messengers). It also entails that one cannot make an oath to anyone else (such as by Muhammaf, by Isa, by Buddha, by Krishna, by Husayn, etc), you cannot make dua (invocation) to anyone but Allah (so not to an idol of any kind), Allah is the one to whom you seek guidance in the afterlife (so you can't say 'oh Muhammad save me from the hellfire, etc').
Tawheed ar-Raboobiyyah is that Allah is the one to whom the Dunya (world) belongs and is the one who gives and takes. Allah is the one who is singled out in terms of sustenance.
Tawheed al-Asma was-Sifaat is describing Allah with the names he himself used in the Quran. Basically saving for Allah the best names and descriptions (Alpha and Omega and such) and not giving these names to anyone else and not taking them yourself (so you can't say I am the one who gives and takes life).
Tawheed al-Haakimiyyah is that Allah has no partners in terms of ruling (Hukm) and he alone is The Hakam (the giver of justice or arbitrator) and he alone is the Tashree (the legislator). This means Allah and his Shariah is the law for all Muslim and Allah is their Hakam.
* there is despute on whether Haakimiyyah is separate from ar-Raboobiyyah, it is a point of debate within the Ulema, but through history most have assumed Haakimiyyah is separate.
The 10 types of Kufr
Kufr ul-Inad disbelief out of stubbornness
Kufr ul-Inkar disbelief out of denial
Kufr ul-Kibr disbelief out of pride
Kufr ul-Juhud disbelief out of rejection, acknowledges Allah but rejects with the tongue and limbs
Kufr ul-Nifaq disbelief out of hypocrisy, as in a Munafiq (hypocrite) who says they are Muslim but in their Aqeedah (condition of the heart) they are not
Kufr ul-Istihal making what Allah has made forbidden permissible or vice versa, so saying alcohol is permissible for me
Kufr ul-Kurh disliking Allah's commands, so saying I refuse to wear Niqab because it is just ugly or such
Kufr ul-Istizaha is disbelief out of mocking Allah or his messengers
Kufr ul-I'radh disbelief out of avoidance, as in one who just looks the other way of Allah and doesn't acknowledge him
Kufr ul-Istibdal disbelief out of substitution of Allah's law, as in a ruler who changes the Shariah rulings of a country or changes a law from the Shariah to something else
Shirk is idolatry which is part of Kufr which is disbelief.
The Jews and Christians would still pay Jizya as the Quran commands it, change the ruling and make the changes to their faith and they wouldn't be Dhimmis any longer.