I think it may be likely that Demandred has actually replaced Roedran himself. Consider what we know:

1. While visiting Graendal in Arad Doman, Sammael comments that events in the South had Demandred's mark all over them.(LoC,Ch6)

- Murandy is certainly south of Arad Doman.

2. Roedran hires Talmanes and the Band of the Red Hand to stay in Murandy so that he can consolidate the kingdom. (TPoD,Ch18 )

- He had never shown any ability in the past and suddenly he's as wiley scheming as Elayne? This is exactly the same play she makes with the Borderlander army in Andor.

3. King Roedran builds an army in Lugard. (WH,Ch9)

- As of the Gathering Storm, it has been stated that Demandred is "gathering for war" and that his "rule is secure" as if this is a relatively new development.

4. King Roedran is so impressed with 'Fog and Steel' by Madoc Comadrin that he has two copies printed and told Talmanes that it will make him a 'Great Captain'.(KoD,Ch26)

- We already know that Roedran is not currently considered a 'Great Captain' and that Demandred's history suggests that he certainly would be considered such. He was the second best general while in the Army of the Light to LTT, and became the DO's best general when he changed sides. 'Fog and Steel' will be Demandred's cover story for why Roedran is suddenly competent and then some.

Just a rant that I decided to look into when I read Etzel's post. I thought he was pretty much spot on.