1) So the defeat of Byzantines would result a brain drain towards Italy? I think that's fairly likely. But for Italy I don't think it would make very big benefits for Italy. The Muslims had before occupied Egypt, Syria, and Mesopotamia which were kinda the intellectual centers (that were way more important than Constantinople as it) of the east Romans. Did that result in a brain drain towards the rest of the Byzantine Empire ? In fact while Italy would may going to be a bit stronger, now for the Muslims who has already taken all of the Hellenic cultural centers would going to be even lot stronger, if only culturally. And I fail to see why brain drain towards Italy is inevitable, based on my second line.
2) Maybe. But those Arabs IOTL had managed to stay around in Iberia for about 700 years or so, no ? Without adapting their way of war to suit their terrain needs, could have they managed to do this ??
Simply you were jumping into conclusion. Warfare isn't the only issue, political situation is more likely the primary. I fail at understanding why they wouldn't be able to adapt their style of warring.
3) I may have failed that. But I'm afraid you have missed an important thing. The Spanish Christian fiefs were not alone IOTL. a) There were the volunteers from other parts of Christian Europe which had been coming to help them. b) Not to mention that the Franks under Charlemagne had also contributed something. With the at least the death of Charles Martel, which will negate the birth of Charlemagne, that would cease both a & b from happening ITTL. Maybe at least the a) would be delayed or so if Charles Martel only died but the Franks weren't really or exactly lost. But if they were lost as well, both a & b won't going to happen at all. With out the help from fellow Catholics, things would be harder to face by the Spanish Christians. And if the Muslim eventual expulsion of the Muslims from Spain IOTL was by no means inevitable as it was a 7 centuries long term process, ITTL where Christians are screwed, it is even further from inevitable line than IOTL, Muslims are going to be driven out from there, that is. Western Christians being limited in sporadic areas will be the BEST result of this from Christian's perspective !
4) The caliphate was about very near the absolute line but not yet on that. They certainly could have been achieve their absolute strecth without the emergence of Leo the Isaurian and the lost at Tours.
Couple of problems:
1. Keep in mind that iconoclasm also caused a huge schism within the Christianworld. No iconoclasm, much more unified eastern Christianity.
2. Killing Martel and Charlemagne, you wouldn't necessarily preclude the rise of a Frankish war leader later on. Remember the Muslims were extremely disunited when the Christian crusades came about, and Saladin still roe up to unite many of them a century later.
3. You just can't kill off an established religion beyond the first two centuries (years of rice and salt is crap; there were, by the time of the black death, still huge JChristian minorities in Egypt and Syria). The papacy survived the freaking collapse of the West Roman Empire. The church also survived three centuries of far worse persecution than the Caliphs were ever going to throw at them.
4. Let's remember that, even after a lot ofhuge advances, the Caliphate still eventually fragmented. Fratricidal local struggles will give European Christendom (and maybe even Greek) a chance to regroup, just as fratricidal struggles among the crusaders, and between the crusaders and Greeks, did with the Muslims. You just can't severely weaken a religion over 700 years after it's founding, unless it's a state cult and the state converts (Zoroastrianism). Same thing would go for Islam pretty much after Mohammed's death I suppose, though you might be able to reduce it to a regional religion during the very early years of the rightly-guided Caliphs.