1) Why do I think moving the Stone is reasonable? Because the Caliph-figure Mikhail in Egypt controls the Holy Sanctuaries. If there are rival Caliphs perhaps you'd have different "flavours" of Islam, and radically different Schools of Law developing, in each 'Islamic Jurisdiction'. If that is so, and the army is behind Mikhail, then he could simply exercise his temporal political power to bring the Stone to a new centre. He could prepare the way with lots of propaganda preached from the pulpit of the Al-Azhar-analogue. For example, he may accuse the Guardians of the Holy Sanctuaries of being wicked and corrupt and not fit to protect the Kaaba any longer. Then, if Mikhail were determined, he could have certain rebel tribesmen invade the sanctuaries and desecrate the Holy Sites. In mock anger, Mikhail could then mobilise his forces, crush the rebels and 'liberate' the Stone from the 'clutches' of wicked pagan "jinns and shaytans". You may think this is an unlikely scenario, but consider: Islam by conquering Constantinople will already start to come under new influences, which may make it deviate from the purity of OTL Islam. And just how powerful would a 'factional' Caliph be? I think enormously so; he could make such changes if he really wanted to. Maybe, too, the concept of the Umma would be weakened and diluted if you have a Caliph in Egypt, another in Rum/Anatolia, another in the Persian East, another in Iraq etc. Of course IOTL you had Umayyad Caliphs in Spain, Abbasid ones in Iraq, Fatimid ones in Cairo - but the point is there are numerous ways things could've gone; Caliphs as semi-messianic figures, with military, religious and political support for their actions, could do these sacrilegeous acts. This is alternate history; you can argue that how things turned out IOTL was never inevitable; the possibilities and permutations are endless.
2) Thank you for this quote. It is very interesting. 'Mother of Cities' is a poetic term and not a clear one like 'Mecca', which would leave absolutely no room for doubt. This may seem like a basic question, and I honestly do not know the answer, but how do Muslims know this is a reference to Mecca? Perhaps there exist traditions of interpretation going back to the very beginning of Quranic exegesis? Probably that is so. I defer to you on this; I would be fascinated to know what sources of authority laid down this interpretation. I ask, not to be mischievous or pedantic, but because I genuinely thought both Christians and Muslims considered Jerusalem the omphalos, or navel, of the earth. One can make the argument, at least, that the 'Mother of Cities' is Jerusalem - and that in the Sura you quote above the Quran could be warning the people of Jerusalem, and surrounding areas of Palestine, who were Christian in the time of the Prophet, of the 'Garden' or the 'Blazing Fire' to come.
3) As for Mecca being the geographical centre of western, or all, Arabia in the 7th century, I thought it was Yathrib which was the senior of the two cities - and Mecca only gained the ascendancy during the time of the Prophet. I think Mecca was the religious centre - the place with all the idols which the Prophet broke - but Yathrib was the wealthier merchant centre with its powerful Jewish families and allied Arab tribes.
4) The thrust of your original post seemed to imply the 'unreasonableness' of moving the Stone out of Mecca was so ASB as to have made it practically impossible to contemplate in almost any alternate history scenario. Now you seem to backtrack from that position. I think we are actually closer to agreement than you make out.
5) I don't know, really, because the Mutazilite position was so totally defeated IOTL Islam. I think what I'm trying to say is that maybe an Umma steeped in ancient philosophy would have argued that each Muslim could have the freedom to interpret the Quran in a more metaphorical, or allegorical, way for themselves. Instead of the Literal Word of Allah, maybe the philosophers would have advocated that the Quran contained the Mysteries of Allah, requiring great learning, contemplation and advanced religious understanding to be interpreted - i.e. a sort of 'quasi-priesthood'. I'm thinking indeed of the Jewish rabbinical tradition, which, as a poster on another thread pointed out, was shaped in this way by Rome/Christianity.
Or maybe, 'Western Islam', as I've postulated it, would be the Shia analogue - with powerful priest-imams interposing themselves between Quran and Muslims - and an overall "Messiah"/Pope-type leader. Maybe 'Sunni Islam' would have developed elsewhere, differently, or maybe even not at all. Perhaps on top of all this a third variant of Islam would have emerged, featuring 'Sunni' elements, but passionately believing in the bloodline of the Prophet - in other words, maybe mystical-messianic Islam would have come to dominate and classic Sunni Islam be in the minority.
If any of the above happened, then I think Islamic Law could have been radically different from what has come down to us.
Would Roman Law have "forced" Islam to have a 'secular' element to it? Maybe yes, maybe no - it depends what type of Islam emerged in the Roman Lands. I don't think it's likely that "pure Sunni Sharia" would have come into being if it had taken over Constantinople and the Roman Lands - "pure Sunni Sharia" emerged at least partly because its underlying society/culture was not a Roman one - it was a unique blend of Arab, Persian, Jewish and Roman elements (with the Arab element dominant for the crucial formative period of Caliphate history).
I don't comment further on the Suleymanic Kanun, Quranic scholarship in relationship to law, or Hadith IOTL because they are what they are because of history as it actually was. What they can tell us of alternate-history Islamic Law is very uncertain. But this is my basic point: to postulate alternate-history Islamic Law you must deconstruct what is, and imagine unusual departures down unexpected pathways.
6) Yes, this paragraph demonstrates what I am trying to say: i.e. from the same corpus of Revelation, you get the radically different developments of Shi'ism. I argue: from the same corpus of Revelation you can have many more variants of Islam - some of which, to come back to the original point, could have been led by a Caliph-Imam type strongman, who could conceive of, and actually implement, the moving of the Black Stone out of Mecca.
1) Moving the stone is possible. It happened IOTL. But as I've said building a new Kaaba is nothing un-serious. It is possible to happen
(but seriously unlikely), but your suggestion that that is for requiring Hajj revenues is quite anachronistic, mainly because during this early days most of the muslims reside in Arabia, which will going to have problem with the New Kaaba that resides in a more distant country, besides the general rage upon this heavy apostasy. AND I also suspect you of generalizing the conditions of this time frame with that of the present day, where there are already more than 1 billion muslims, though yes Hajj pilgrimage has been a revenue maker for a long time IOTL, but to be frank I'm personally not sure it was already the case during 9th century....(Note also that, unlike ITTL where balkanization of Caliphate happened earlier, IOTL during this time it was still intact under the Abbasids.)
So say that the ruler in Egypt says, "The Meccan Kaaba suxx, this Kaaba of mine in Alexandria is the real one", what will happen next is that the Arabians (which ITTL is currently under the rule from Alexandria), especially the Hejazis will reply,"fcuk you very much, dick wat", and rebel. BTW isn't there ITTL a rival faction to the one in Egypt which only a Mesopotamia away ? Which ALSO claim the title Caliph ?
And also, as I've said, Kaaba has been destroyed IOTL, more than twice ! And it was just rebuilt again after being destroyed. So if the Egyptian destroys it, it will be just rebuilt after the Egyptians are repulsed.
Unless this New Kaaba
is in the whole set of attempts to accommodate the native Egyptians, I don't see any other reason for building it. OTOH, if this Egyptian New Kaaba is actually built, I personally think that this is maybe the chance to form a sort of Islamo-Coptic hybrid religion in all but name only. Maybe even including the name also !
2) Those in Jerusalem can probably make a fake Qur'an of their own to generate something like this, but this will also require them to claim that Muhammad was born and lived in Palestine. Other muslims will going to comment on such a claim.....
3) In 7th century, and that was because of Yathrib's VICTORY over the Meccans, under the leadership of Muhammad himself. But since then the spiritual value of Mecca has become even more meaningful than ever.
4) I said that about building a new Kaaba, not moving the Black Stone.
5) The principals of Mu'tazilism actually weren't perceived as friendly by the conservative clergy on their political power. Even if without Mu'tazilism, muslims are actually to understand Qur'an individually. But however the culture of carefulness when interpreting Qur'an and Hadith away from literalism actually give the clergy political power.
Here with Islamized Eastern Rome, we've got to see the muslims there to be influenced by both the culture of priesthood
and Aristotelian rationalism. Things will surely get interesting from there on.
Though there I'm not sure about what do you mean by "Sunni Islam" forming some where else. Actually the branding of "Sunni Islam" is somewhat unnecessary. It's just that the "Sunnis" are the
Ahl as-Sunnah wa al Jama'ah, the followers of
Sunnah or the examples of Muhammad, in which the Shias can perfectly claim that they are also doing it. In fact, never forget that before the rise of Safavids ITTL and the consequent conflict between them and the Ottomans, the lines between Sunni and Shia were blurry. It is that the Shias have a thing to identify themselves as a group that is different from the rest. The Javanese Abangans also do that as well, btw.
But if you simply mean that Eastern Rome will develop its own interpretation of Islam along the lines of their culture, well that will be certainly the case. The Acehnese did, the Moroccans did, and they are both "Sunni" people. Sunnism is about how coherent or contending your interpretations of Qur'an and Hadith with what texts in those say, and there are be many ways to claim such.
So Islam in Eastern Rome can be either Shia or Sunni or even a Greek Abangan(but this last one is hard. It isn't as distant as Spain), but that doesn't mean that Islam as whole will immediately changes. However, if the idea of Shiistic Imammate wouldn't sip into them, then Eastern Romans will most likely be just Orthodox Sunnah-Oriented with more powerful and organized ulemas, ala Jewish rabbis. I'm not sure if the clergy will be sentimentally in the opposite side of the river with the literalism tendencies. If anything I see
the resistance against them that will oppose literalism.
6) Again, the moving of Black Stone is possible, if will generate some rage. It's simply that a new Kaaba will be to hard to swallow.