I'm ready to believe that the story had little or nothing to do with the eventual Sound-driven cataclysm. It may simply have been a glimpse into the mindset of Emet and the other Ancients, highlighting their unmalicious callousness when it came to lesser beings (a callousness which would later be inflated to the point where they were willing to do what they did to usher in the Rejoinings). For now, though, I believe that the Phoenix episode was a hint at the origins of the Sound and the End of Days, and how even the greatest among the minds of the Ancients failed to recognize the threat before it was too late.
It's neither. The episode is summed up with this.

"Consumed by the fear of death, it thrashes blindly about. It will know only pain and suffering and inflict the same upon others. A pitiful existence."

"Such moving empathy. It's as if you wore the feathers yourself."
People keep thinking there's something untowards with the phoenix that hints to the start of the Final Days and the Ancients causing it, but the truth is the point of it was to draw a direct parallel between the bird and Emet. I don't know how it could be made more clear, considering Hythlodaeus' line.

Eventually, yeah, they turned to these desperate plans. Earlier on, though, it appears as though they were less concerned about it, hence the "debate" among the Ancient NPCs about whether Amaurot should go to the aid of the far-off suffering cities or not. Once the problem was on their doorstep, you're right, they started thinking long and deep aobut what to do. (At least, as long and deep as they could with monsters literally springing out of their heads left and right and fire falling from the sky.)
I don't really think the debate between a couple of average citizens is all that indicative of what the authorities in Amaurot like the Convocation were thinking about it at that time. Given dealing with affairs in the wider world was the primary purview of the seat of Azem, the rest of the Convocation was probably not prepared to just jump in wildly. Moreover you suggest that they should have acted sooner, but part of that debate you mention addresses that Amaurot really isn't supposed to be some saviors of the world. It's precisely that thinking, which was ultimately brought about by everyone else being wiped out, which led to 12,000 years of Calamities with the Ascians trying to reunify everyone's souls and save those who had sacrificed themselves. In fact you could say this debate was the core of the conflict between the Convocation and the Anyder, as the former took up the mantel of stewards of the star while the latter effectively stood for non-intervention beyond the point of pure survival.