1. In the Interstellar Wars era (especially as detailed in the GT sourcebook), the Ziru Sirka was rife with internecine struggles between Vilani Great Houses. It is inconceivable that the Ishimkarun would be anonymous, when most of the province-level leadership were acutely aware of the need to maintain and expand personal and familial prestige.
This was the Ziru Sirka in it's senility. I can easily imagine that by this point in the general decline, the Ishimkarun had become a mere figurehead, with real power long since claimed by the Great Houses. Nobody much cared anymore who held the Shadow Throne, since that position had literally become a shadow of it's former importance.
2. The Rebellion-era Ziru Sirkaa had no hesitation about appointing as its first restored Ishimkarun the very high profile (and member of a pre-eminent Noble House) Archduke, rather than than appointing an anonymous technocrat.
Here we have precisely the reverse perceptual situation. The 1100+ year example of the 3I's Emperor had gotten everyone used to an actually-powerful central ruler. At least, a ruler reputedly powerful enough that the entire cultural beat itself into shattered fragments trying to fill said position with their own partisan choice. So while the title may have been "Ishimkarun," that actual position was nothing like the original one.
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Richard Aiken
"Never insult anyone by accident." Robert A. Heinlein