Also note that the conversation also implies it takes place post-Zodiark's summon, so they had evidence it didn't work too.
We have no idea what their reasoning was.

Dude, they planned to kill everyone. All the life that currently exists means absolutely nothing to them.
I was referring to pre-Sundering and what life they intended to sacrifice. Furthermore saying that all life means nothing to them isn't really accurate because the entire point of the rejoining is to rejoin all currently existing life back into it's pre-broken forms.

I agree with this theory 100%. This also explains how Azem, who isn't present for the summoning of Hydaelyn and Zodiark, can post-sunder rejoin this group and rebuild the world. Lets not forget that Nald'thal are twins who count as one god, which make "the 12" actually 13--the 12 Ancients who followed Venat and Azem.
The theory doesn't work because Azem wasn't a follower of Hydaelyn and didn't support her summoning. And given that WoL is a fractured soul, Azem was either sundered along with the rest of the world or somehow survived only to be sundered at a later date. If anything it's most likely the Twelve are just half-recollections of the Convocation, or even simply all interpretations of various incarnations of WoL throughout the ages.