The Iran series: reaching terra firma amidst the historical quicksand #2

What was the nature of the conquest of Iran in the 7th century?
Was the Sasanian empire as centralised as we often think?
A curious parallel between Sasanian revisionism of the national narrative and that done by the Abbasids, during a similar timeframe.
Can the narratives of the Arab conquest of Iran be trusted? Do they offer a dependable historicity?