Note: This should occur sometime during the late 6th or early 7th century... i.e. cannot butterfly Arab Islam or the early years of their conquests.
In the event that the Sassanids successfully take and hold Constantinople and begin the steps necessary to drive the Byzantines out of Asia while gradually transfer the seat of Sassanid power to Constantinople...
1. Could the ERE set up a government in exile in Rome? Assuming the conquest didn't kill off the political and military leadership in Constantinople, can they expect to move into Rome and re-establish their seat of power there?*
2. How gradual could a concept of Constantinople being the capital of Sassanid Persia take hold? It's a good capital to have and the Sassanid leader would know this, but how long until the people of the empire think so?
3. With both the Romans and Persians even further beaten and thinly stretched than OTL, how much easier can the Arabs expect to be in expanding west and northward? Can they take a weakened Sassanid Constantinople ttl? What about Egypt and Mesopotamia?
4. With the ERE based in Rome, what does European history look like into the early middle ages?
5. Short and long term effects? Most interesting way this could play?
6. If not Rome, then where?
Thanks in advance.
*Keep in mind, that it would be nice if the ERE keeps its holdings in Europe and North Africa (minus Egypt), while the Sassanids keep everything in Asia/Middle East until the Arab conquest. While both parties are going to be even further stretched for manpower and resources than OTL, it'd be nice to establish at least temporarily Roman Europe and Sassanid Asia/ME with the Romans having a keen eye on revanchism just in time for the golden age of Arab expansion.