Demandred with the SF has several weaknesses.

1. They aren't a relevant military force
2. They have been split up since KoD to delivery supplies where Demandred seems to be in the middle of a consolidation
3. They are not strategically well placed at the moment
4. They are a Naval force with no one to fight

Even if Demandred isn't Roedran, I have a hard time seeing him being among the Sea Folk. I think what you have put forward is for the most part well reasoned and solid, but RJ always had a purpose for where he placed characters and Demandred among the Sea Folk seems to lack that ... Nearly all of the Forsaken (Semirhage, Mesaana, Rahvin, Be'lal, Sammael, Graendal, Aran'gar, Osan'gar) were attached to major Political/Ruling bodies, those that were not (Moghedien, Asmodean, Lanfear, Ishamael)directly involved themselves with Rand and company (or in the case of Moghedien Nynaeve).

Demandred is clearly not working an angle with any of the main cast of characters or he would have been spotted by now (either by the readers or Graendal who keeps tabs on the TR group). That leaves us with limited options to pin him down.

1. Roedran
2. Borderlanders
3. Seanchan
4. Sea Folk
5. Aiel

There don't seem to be any good candidates among the Aiel, Borderlanders or the Seanchan since RJ and BSand ruled out anyone who has appeared "on camera" prior to AMoL ... which IMO also rules out Amel who, did have a cameo, even if a brief one.

Roedran is well placed strategically and with a consolidated Murrandy has a significant power base to open hostilities ... Andor is in a weakened state thanks to the Trolloc's invasion, which Demandred can make worse by provoking a conflict with the Seanchan. Even if you throw out a human army, He is in a prime location as Roedran to direct armies of Trollocs coming out of the Ways all throughout the continent.

Personally I think Demandred having an alter-ego is kind of lame, he doesn't need it at this stage in the books and it would be much more interesting to me if he simply appeared with dozens of Trolloc Hoards coming from various Waygates to smash the southlands.