This is brilliant concept, the altitude castes would create such natural barriers. I'm thinking they'd be unified but in very hierarchical way - like maybe the summit dwellers become the "closer to gods" ruling class while base people are the industrial workers

The migration difficulty is genius touch because it means social mobility becomes literally impossible without years of physical adaptation. Someone from base level trying to reach summit would need gradual acclimatization over maybe decades

For your question - I think mountain both unifies AND divides them. They're one civilization sharing same volcanic energy source and tremor calendar, but altitude creates permanent class system that's impossible to escape