And to circle the topic back around to the OP: for what it's worth, I got my hands on a Japanese translation of the codex text, and unsurprisingly it made no mention of the Convocation ordering anyone to sacrifice themselves to Zodiark. All it said was:

"To summon Zodiark, approximately half of the survivors of the Final Days offered up their lives as sustenance, while the representatives of mankind, the Convocation of Fourteen, performed the ritual [lit. art/ magic."]

So there you go.