Why do they have to destroy each other? Maybe Shauriatas is undone as Shauriatas but is now living in symbiosis as a new combined entity with the Dunyain. Maybe it's a mutual arrangement. It would explain how they could do sorcery, in any case. IIRC all the Dunyain with that potential murdered themselves in the prologue of the first book. But if they have a sorcerer's soul being utilized between them, maybe that's not an obstacle.
I don't think it was the ones with the potential, just the ones he contacted, and as I remember they were all elders. This makes(for me at least) a new question: Who can receive dreams? Can it only be the Few? We know almost nothing of the Pshuke(misspelled?), and so we don't know how those Cants of Far-calling work. All we know is the ones contacted killed themselves, and the others honored their sacrifice by not snooping.
And I firmly believe that the Dunyain would be capable of sustaining Shauriatas AND acting Dunyain normal. The way they all came down their own steps, and stayed there, was kind of telling. And Shauriatas ASSISTING them with Cants is a cool twist to my initial thoughts...Still not sure if this is what happened, but if it did, I can see it.