Do you think that the Twelve will eventually become major players in the plot?

[Risvertasashi]

I'm not saying the twelve ain't real. I'm saying that they likely won't appear in a physical form without the Ascian's consent.

I don't think their consent has anything to do with it. They teach people how to summon primals because it's often in their best interests, but there's no indication anywhere that Ascian support is required.

Are you absolutely sure about this?

Technically, without a dev answer, we can't be absolutely sure of anything. I'd say it's improbable they had anything to do with planning on the Twelve being summoned. But I find it particularly unfeasible/unlikely they had anything to do with the summoning of Phoenix. Shiva also seemed to be an independent operation--although the Ascians are involved with Ishgard, Iceheart is their enemy, so...