So as zoroastrianism was used to unite iranians, nestorianism was a tool to rule over the christian semites of mesopotamia. So basically this implies that the 7 parthian clans were not at all orthodox zoroastrianism and that they espoused heterodox antiquated religious traditions. I thought this was the role zoroastrianism played considering they attempted to convert armenia and assyria.
Perhaps the suppression and destruction of Iranic paganism assisted the muslims when they conquered the Iranian world. Perhaps the Sassanids already completed much of their work.
Considering the danger the Kidara, Alchon and Sveta Huna played, this would make sense as they considered themselves successors to the Kushan.
I am not sure that one may define the issue between the Sassanid monarch and the 7 great houses as one of religious nature. Though some of the houses likely did practice Iranic paganism and or Mithraism, their issue was different. The issue between the imperial house and the houses were that:
1. The great houses claimed a supernatural investment of the lands east of the Zagros as their domain. That is, a sort of Donation of Constantine situation, that they as large land owners had an ancestral right to rule these lands under the Emperor. They were thus, forces of decentralization no matter their faith.
2. When Peroz I was defeated and slain by the Hepthalites at Herat in 484, the Great Houses uniting their efforts, appointed the new emperor (Bolas). This appointment came too, with the Great Houses defeating the Hepthalite horde in battle and saving the empire. From this point onward, the houses had set a precedence of appointing emperors and also of saving the realm where the emperor had failed and been slain.
Later emperors saw the weakness in allowing this situation to continue. Hence the intensification of the conflict between the great houses and the central imperial court in Cteshipon.
Nestorianism was a tool not only to rule the Semitic populaces, but as a tool and model to empower the Sassanid monarch. It is not as simply as Semitic = Nestorian,, many of the Semitic populace of the Sassanid empire continued as pagans or were Manichaens or various other similar faiths.
I would not say the Hepthalites and their conglomerates were/are successors of the Kushan. However, the geopolitical role is slightly similar, yes.
Yes, I would agree that the Sassanids had done much of the work for the Muslims. The destruction and subversion of traditional culture and the play between the monarch and nobility allowed a more thorough top-down transmission of culture and religion to occur than what would have been possible otherwise. This is why Zoroastrianism in the sense of the general established religion played such a meager role during the caliphates. They were so tied to the Sassanid throne and Sassanism, that it ceased to be a power after said empire was lost and its memory extirpated.