What is, IMHO, impossible, is to have a continuation of the OTL Arab-wank once Constantinople has fallen. It's a lot more difficult for the Arab armies to fight through the Balkans with the fearsome Slavic and Turkic tribes swarming through the area, than it is for them to take over the relatively well organised plains of North Africa. Indeed, I'd say a Caliphate that takes Constantinople is likely to be substantially smaller- it'll take Thrace and roughly the borders of modern Greece without much of an issue, but resources are likely to be diverted, which will allow the survival of some form of Christian state in both the Exarchate of Africa, and in Visigothic Spain.
I disagree on this. Maybe not in the Balkans, but:
1) Conversions are quite possible (as they were in OTL from Constantinople). I wonder if the Magyars turn Muslim...
2) I can't see Sicily and Africa having the resource base to hold the Muslims at bay. In OTL it was a grinding conflict for North Africa; but in the ATL, without Constantinople, where will the resources from from? It's not like the Lombards will let up in Italy because Constantinople fell.