We have two sets of paramaters set by Jordan.

First by the mid to late series he answered a question about it that we could figure out who both Demandred and Mesaana were hidden as.

We know from the few clues about Danelle that he was telling the truth. We could get lucky, spot the clues and figure it out. It didn't rest on solid facts, but we could still suspect it was Danelle based on flimsy clues and turn out to be right.

The same must be true for Demandred.

That rules out Amel, who was never mentionned before KOD. And he rose to prominence following a series of events Demandred could not possibly control. A lot of your "suspicious" stuff about Zaida isn't suspicious at all, you just overlook politics. Had Mat seen Harine, she would have looked as acting just as weirdly as he found Zaida to be acting strangely. It's not any deep mystery: women of Zaida and Harine's rank are almost never seen. They're the next thing to the Mistress of Ships. It was unique that they lead embassies on the land, and the reason for this exceptional event was plain and simple, they were the embassies to the Coramoor...

You overlooked the political contest between Zaida and Harine. Both had to be the one succeeding at making a Bargain with Rand. Harine won, but her Bargain was very onerous and Zaida got the advantage of learning earlier about the death of the MoS and to play all this against Harine.

It's as simple as that. The Sea Folk have no real army. They never needed one, their islands are almost unreachable except for their own ships, and if attacked at sea or at home, they can fall back on Windfinders and leave no survivors to tell the tale... When organized military faced them, they were fairly easily defeated. They lost an island to the Seanchan, and at Ebou Dar they did damage by surprise but the damane got the upper hand pretty fast. The husband of a captain (by whatever name) is in charge of Trade and protecting the goods and the traders. What you call "an army" is more akin to merchant's guards. He has not authority whatsoever over the Windfinders, who are strictly under the female hierarchy.

It's not enough for Demandred to "have an army". He needs a way to either turn it against the Light (fat chance of that with the likes of SF, Aiel etc.) or use them in a proxy war. Aside from the Seanchan, the Sea Folk have no enemies, and they're proven not to be any match to the damane, so what would be the point?

The other point is that Amel, if not by name, has showed up on screen in that scene where Mat saw him. Jordan said we never saw Demandred in disguise on screen (that was valid up to KOD). Brandon confirmed we haven't seen him in TGS/TOM either.

There are few, very few characters fitting the criteria, and the only one who makes sense among them is Roedran. I've fought against the idea for years but have given up. The clues are slightly better than those for Danelle, but they are there. Toward ACOS (or POD?), reports that Roedran had changed and was becoming clever showed up. He's developped an interest for old war treatises (that's Demandred learning all he can about third age tactics...).

All that was left to make sense of this is to figure out Demandred doesn't intend to use the Murandian to get himself an army. The Murandians won't fight for the Shadow. He intends to use the Murandians to start a conflict detrimental to the Light, and draw into it as much of Rand's forces as he can. Demandred will provoke a War between Murandy/Andor and the Seanchan. He will offer his armies to Elayne to deal with the Shadowspawn and make an alliance for them to act together should the Seanchan attack either of them. Then he will bait the Seanchan into attacking him (there are dozens of ways he could achieve that), and draw Elayne, her cannons, the Band, her armies into that conflict. This is the biggest single force of the Light distracted at the wrong time. Once he's dealt with the Seanchan or during the war, Demandred will seize Elayne, unveil himself and draw Rand's forces into the maelstrom, the Shadow's armies ready to strike.

The flaw in Demandred's plans is the Prince of Ravens, he's not expecting that Mat through events will rise to leadership of the Seanchan armies. The Light will survive the "Great Battle", and out of it the Seanchan and the other half of the Light will emerge as allies against the Shadow.

Demandred already has armies: millions if not billions of Shadowspawn, with plenty of Myrrdraal ready to force them to follow all his orders. He no doubts has plenty of those hidden in the South, ready to strike at the Seanchan in Ebou Dar and at all their other military posts (Illian, Amador etc. as soon as they commit enough troops to his "Murandian War".

If he has Dreadlords serving him, they won't be Taim's, The BT is a decoy within a decoy, an army hidden within Rand's army but not well hidden enough... Demandred's forces will rather consist of a selection of Taim's recruits who have not joined the BT but have gone elsewhere after testing positive, and who now have joined fairly recently the Legion of the Dragon. A force of well-trained male channelers ready to strike by surprise at his orders, and right among Rand's forces, and lead by one of his best and most valuable general (one who has "something dark" in his future...). We'll see if the BA will join his channellers after their betrayal, or if they have been merged with Taim's forces (the latter, IMO).