Don't you see the huge flaw in all your reasoning:

How could Demandred possibly convince the Sea Folk to turn against Rand and attack the Westlands'c coastal cities?

You think Zaida would last long if she ordered this nonsense?

The whole Sea Folk culture rests on Trade. They avoided any and all wars with the Westlands for 3000 years, their ships are welcomed in every city as neutral, even when the nations are at war.

The only force the SF have attacked is the Seanchan, and because the Seanchan striked first.

And even then they've avoided letting anyone know of this war. They've not sought allies, they've tried to hide the fact they were fighting.

The leader of the group of Windfinders Zaida bargained over with Elayne were catastrophied to be asked to perhaps be seen fighting with the OP on Elayne's side of the succession war. Their whole trade empire rests of their neutrality. If they tried to conquer Westlands cities, they're doomed. If they turn the nations into enemies, the nations will turn to others for Trade. The Sea Folk trade empire rests on their neutrality and the fact only them have access to faraway precious goods. If they do anything to antagonize the nations, they will use Gateways to go find those precious goods at the source, they no longer need ships to reach those faraway producers. The Sea Folk would be pushed back to the Sea and no longer welcome in the Westlands harbours. They already lost access to Tanchico and Ebou Dar and their lucrative trade.

Come up with a credible way Demandred could turn the Sea Folk against Rand and the nations over the next few weeks of story when it goes against all their interests to do so, and we'll see. In the meantime you have a theory about Demandred's identity and no credible theory when it comes to the use Demandred might make of the Sea Folk in the LB. It's a pure loss of time to discuss the theory about identity until you have a good theory on the goals to offer. For now you have nothing but this nonsense that Zaida will order her forces to attack Rand's coastal cities all of a sudden and the Sea Folk federated clans will follow her in this idiotic venture that would totally doom the Sea Folk.

Don't forget, the Sea Folk believe their future glory rests on serving first the Coramoor in the LB. That's what their prophecies herald, and that's why the made that onerous Bargain with Rand and won't let it go.

Darius's right, by the way. Rand asked them for most of their fleet, all their biggest and fastest ships. The rest Zaida sent back to their islands to try to stop the Amayar's crisis, and she wasn't best pleased she had only slower and smaller ships left for that. The "big ships" left Tear and Illian right after KOD, and have been stuck in Bander Eban's harbour since as they were bound by the Bargain to make that delivery and the Domani wouldn't let them disembark the goods until a few days before the Merrilor meeting.