Constantinople was huge, as a military point. Huns, Avars, Arabs, Bulgars never really managed to really threaten it, so...There might be an intermediate scenario, in which the goths invade Greece and parts of Asia Minor after sucessfully besieging Constantinople.
Your best chance if eventually to get rid of Constantinople and to make Nicomedia the eastern "capital" and pray really hard for ERE to loose its naval advantage.
As far as I know, Rome wasn't destroyed and loosing its symbolical rome before the Romano-Gothic wars in Italy, that ravaged the region including Latium and the City.Thus we would have both Romes destroyed by the barbarians and as a consequence both centres of power fatally weakened.
Strictly speaking? I agree : Parthians were after all overthrown by Sassanians at this point.Parthians do not need to play a significant role.
More seriously : yes they do. Arab threat was mostly "dormant" and unable to take on Romans. IOTL it asked for decades of harsh war in the region to allow them taking it over as a rooten fruit, and this region was IOTL the most dynamic of Romania.
Not even considering, of course, that Vth Arabia was largely under Sassanian influence at this point.
You won't have an Arab unification coming out of blue, and a sudden attack against Romans on their best heartlands, not without major PoDs in the peninsula.
Actually, you'd still have these. No council doesn't mean the arguments wouldn't be made by theologians, scholars and clerics. It's just that these arguments won't be institutionalized and organized trough council and forming really distinct churches : the borders would be more blur but oppositions would still exist.What we do not get, however, are the mostly futile distinctions between Nestorianism (extreme dyo-physitism) and Monophisitism, which developped during the Vth century OTL.
Which is really, really wrong in spite of being repeted on this board.And they developped only because the only possible expression of dissent in the ERE was religious controversy form that point onwards.
Quoting Vincent Deroche, one of the specialists of the question.
Geography of monophysism get quickly stabilized : coptic Egupt is mainly monophysist with only the urban and hellenized minority supporting Chalcedone; Palestine is chalcedonian since Jerusalem beng raised as a patriarchate, not without violent conflicts in the Vth; syriac provinces are mainly monophysist but not crushingly so at the contrary of Egypt with a similar distinction between urban, hellenized and chalcedonian population, and rural arameans and monophysit populations.
At first the monophysist resistance is tied to theological tradition and tensions due to the rise of the Patriarchate of Constantinople before Antioch and Alexandria; Cyril is perfectly Greek, Severus of Antioch as other monophysist authors only write in Greek. All feel themselves "Romans", subjects of the Christian Empire and obviously hope turn this empire to the true faith.
One shouldn't then interpret monophysist movement as a simplist religious transposition of a struggle whom true nature would be political or social
That said, and in a latter time
But religious opposition being parallel with the one between hellenized, coptic and aramean population; capital and provinces, that an early national identity and religious identification of an ethny slowly begins to appear [in the VIIth century]
Basileur Giorgos would be able to speak about it more precisely, but I think you're wrong in your affirmation there, or that it needs at least some points to support it.