The only thing I don't like about this theory is that Murandy, even a united Murandy, is hardly a military powerhouse. Bringing Murandy's armies over to the dark side hardly seems like a decisive or war-turning stratagem.

On the other hand, Murandy's central location is very advantageous. Also, if Murandy's army is supplemented by Dreadlords and Shadowspawn, it could launch at least one devastating surprise attack and then continue to tie up most or even all of the light side's southern armies.

Another consideration though, is the attitude of the actual soldiers. For now we can expect Murandy's troops to follow the orders of their new (and presumably impressive) king, and it may not be too hard for him to convince them that an invasion of say, Andor, is a good idea (and possibly revenge for that border shift in POD). But what happens if he starts filling thier ranks with Shadowspawn? Aren't some of them going to balk? Of course, this argument applies to almost any army he may be commanding.

If Darlin hadn't been seen regularly in the series, I'd have suspected him. He was actually the first person I thought of when Demandred said "My rule is secure, we gather for war". Shortly afterward there was a scene where Rand implied that Darlin had been slow in getting his troops ready to come to Arad Doman, and something seemed off about Darlin and the Tairen army throughout the entire book. But it doesn't seem possible that Demmy could be Darlin, and if he was pulling Darlin's strings, then he wouldn't have said "My rule...".

A final point is that we shouldn't necessarily interpret "My rule..." to mean that Demandred is a King somewhere. He might be some other kind of leader (oh, if only Taimendred hadn't been disproved... it fits so well). Not that I can think of any figure of power who could be said to both "rule" and control a significant army now that the Whitecloaks are divided and Taim has been long ruled out.

So Roedran still seems like the most likely choice, barring something right out of left field like Shara.