A slight correction, Bahram of Mihran defeated the Celestial Turks, not the Hepthalshahs, who had already been a destroyed state mostly by the year 590 CE, they likely existed as a disorganized collection of allied states in the region of Ariana or Kabulistan and then also having sway over Zwambinar alongside the local proto-Pashtun peoples. As a rule, the Hepthalites ceased to be a fear for the Sassanids after 562 CE or so. Bahram of Mihran however did decisively defeat the Celestial Turk empire and then not only defeating them in the field, proceeded to pillage and raid their vassals and subjects in Central Asia for some time before returning to Iran.
Homrizd IV jealously then began to attempt to delegitimize his Marshal by excluding him and forcing him to dress as a woman. Traditionally, after the reforms of Kavadh I and Khosrau I, the Mihranid family had a monopoly upon the role of Marhsal of empire. This is why in a sense, Bahram was a Margave-King and a Marshal-Lord. Hormizd IV before using him, had attempted with his own Sassanid standing army drawn from the urban cities of the empire, to defeat the Celestial Turks, leading to immense failure, hence forcing him to call on his Marshal-House, that of the Arsacid cadet branch, the Mihrans.
Regardless, Khosrau II was supported by House Ispahbudhan and it was their idea to sneak Khosrau II into Syria and then seek aid from Byzantium. House Ispahbudhan rejected Bahram's claim because it would seem, they were jealous or that they felt themselves higher ranked. Ipsahbudhan was also an Arsacid cadet clan and seems to have felt its relation to the older house, stronger and more prestigious. They felt that it was their role to control the King Among Kings as a puppet and that Bahram attempting to assert a true royal lineage over the throne, was an affront to the rest of the Houses, especially theirs. House Karen attacked Bahram also, probably for similar reasons. Generally the custom was, that the Houses always will counter one that attempts to put themselves above the others, and makes this a taboo.
Since this is the case, they will seek to create a figurehead or nominate someone whose lineage is not in competition with their own. The Sassanid royal house was seen as low-tier in terms of lineage and lowly in standing as far as bloodlines went. As such, putting them on the throne was not an affront to the others, for the rest remained complete equals, whilst the low clan placed as Kings are simply given a role that none of the other Houses wish to take as it disturbs their way of life and also destroys their taboos surrounding not overtaking the others.
So, Bahram is in a bad position.... The only way for him to survive is to create a coalition and this is what he attempted to do in otl. Namely, he wished to form a coalition with the Bagratuni of Armenia and create a composite government with the lords of Armenia. Once he did this, it would seem that he may have targeted House Zik as his next partner and after this, proceeded to subjugate the other Houses forcefully, which if he defeats them in the field and the Sassanids are totally eradicated, they will agree to his terms and a new confederacy will be formed. A sort of Mihranid Confederacy will begin with the ruling two Houses being Bagratuni-Mihran and then the other Great Houses ruling their lands as they had always done. This Mihranid state would however have issues with Byzantium and will need to prove itself by raiding into the empire and also securing its hold over Aremnia.