I don't think the "shared framework for coexistence" is multiculturalism because that isn't well defined.  I think what keeps people civil is the legal system.  A legal system based on enlightenment principles of freedom and equality naturally supports multiple ideologies and cultures...to the extent that these cultures aren't colliding.  For example, it isn't legal to stone people in the USA even if some cultures do it, but if people want to smoke a hookah that's their business.

OTOH a lot of polarization is created by intelligence agencies as a means of soft power to change a nation's government.  It isn't usually organic or the result of culture clashes.

When polarization is the result of fundamental irreconcilable differences then I think that is Casus separatio and I favor independence movements. 

 Great points (as usual). Laws rooted in freedom and equality do set the foundation for coexistence, but laws alone don’t build bridges. they’re the structure, not the soul or the will/determination to do it. A shared narrative fosters trust and connection in ways the legal system can’t imo.

I agree polarization is often (if not always) manufactured by powerful actors, but why are some societies more vulnerable to it? Isn’t that vulnerability a sign of a fractured or missing shared identity?

And on irreconcilable differences: when does separation strengthen a society instead of weaken it? Curious to hear more. 

 When examining the history of America, the constitution functioned as the shared principles among diverse populaces (consensus rules) — but it was the excitement and story of America that played a mysterious role in ensuring its success. Not mere patriotism, but something deeper. Same with bitcoin and Satoshi. The story paired with the laws.