I would not be surprised he still has the real Roedran under compulsion and well hidden (well away from Lugard, accessible only by Gateway), and he replaces him with a MoM only for important tasks, happy to let the real Roedran have his parties and whatnot. If Demandred is Roedran, he played this right. He's been patient, didn't have some great noble appear to openly gain influence as Sammael, Rahvin and Be'lal all did. He took his time to study Roedran to be able to replace him, and worked to make his "evolution" of personality happen smoothly, as if Roedran was maturing or naturally changing, coming into his own. He's kept Murandy totally out of anything until the AS's presence offered him the opportunity to hire the Band and have a credible, not too suspicious way to unite his nobles around him and "secure his rule". It was convenient, but he had an alternative which was to wait for the Seanchan to mass armies at his borders... He even made sure to let the Band go.

RJ was clever, but the clues are there Roedran has changed, and not all that hidden. It showed up in both the Rebel and Succession storyline, in small touches. The Rebel AS perhaps sent him an embassy as they did with other rulers around, but RJ made sure never to attract our attention on it (they are almost certainly not BA, if they exist. Demandred simply use them to report there's nothing to report, but it looks like there's no Rebel or WT embassy in Lugard, and no advisor). "Roedred" is an old theory, which only gained strength with KOD and Talmanes but that had been around since he hired the Band or so. It just took forever for credible theories as to why Demandred would have any interest in replacing King Roedran and gain himself an army of Murandians, of all things, to emerge. Mine's not the only one (probably not the first either), but most of those I find credible center on Demandred using the Murandians as proxies, not at all as "his army".

Demandred can't have any army turn to the Shadow. If he doesn't have an army of DF like Taim has, then the army of the Light he controls has to be fooled in fighting armies of the Light, and the most harmful such conflict Demandred could engineer is one with the Seanchan, which happen to fit with the geography, his recent dealing with Elayne and it used to be complementary plans of Mesaana and Semirhage. (the other believable option is some plans for Andor, but I found those weaker).

It's not that the theory of the OP is bad, as far as tracking the identity goes, but it hits the same wall the Roedran theories did a few years ago: we can't see a plan in there, except impossible stuff like ordering the SF to attack Tear, Illian and Bandar Eban. It doesn't mean it doesn't exist (though I agree with you Amel himself is ruled out by his cameo), but until someone comes up with a solid and credible way in which Demandred could use the Sea Folk (as it needs to be as proxies, there's no way to make them fight the Light except the Seanchan and they're no match for them on their own, nor located for it) Demandred with the SF is not credible.