What if the Zanj Rebellion had succeeded in overthrowing the Abbasid Caliphate? Could a Zanj polity have lasted?
To be extremely frank, yes the Zanj polity can last. Ali ibn Muhammad al-Dibaj Sahib al-Zanj was one of the greatest political actors of the Middle Abbasid period and of peerless skill in organizational planning.
The Zanj state operated for a decade or so as an independent country in effect and minted coins, created borders, warred independently against many actors, created trade lines, collected taxes and many others. Going forward, the main issue at hand will be religious disputes.
Regardless, I have written much in the Zanj state apparatus and the disputes within it on this site. Though, due to brevity and to allow a discussion to begin, I will reserve some of my points until later.
What kind of political system did the Zanj operate? Was it somewhat anarchist?