The Exarch and Zodiark

[Absimiliard]

My reading of that scene was that Emet-Selch was busy trying to salvage the situation on the First, where Minfilia's sacrifice had brought the First back to "brink of a Flood of Light" rather than "actual Flood of Light". Since there needs to be a major disaster on the Source to coincide with that in order to complete a Rejoining, he likely just (in that moment) had the idea of using Black Rose as a catalyst, rather than whatever plans the Ascians had before that went awry. (Given Lahabrea's involvement before Emet-Selch took over, it's likely this was Thordan.)

The Ascians actually wanted it to hover on the brink. A full blown flood of light consuming the entire world would've had similar results to their first (failed) attempt at initiating a rejoining. The only real difference is that it would be a void of light instead of darkness. Ardbert and his companions not doing what the Ascians anticipated is what actually tipped the scales beyond the breaking point and triggered the flood. The flood is what forced them to step up their timetables on the Source; to trigger a rejoining before the First could be completely consumed by the light.