So lets assume the shadow take all of Rands cities as you say, sneaking through the back door if you like.....

1. So now the Light have a super massive army of awesomeness in the middle of a field somewhere up north. The Seanchan then show up in full power. Rand snaps his fingers (or whatever)and they are bound to him. Now they really have 1 awesome army.

So, what does he do? Get Mat to co-ordinate retaking all of the shadow controlled cities, whilst Rand goes with a small force straight to SG? Even if they went en-mass to each city in turn in overwhealming force this would take months. They are going to have to fight street by street, house by house. How do they assault the cities? The shadow forces will hurl live humans at those assaulting the cities and eat the rest. Although it's not a bad plan from the shadow I cant see this happening, for 2 reasons. Firstly, it's pretty much complete defeat for the lightsiders even if Rand wins. Second, I'm not sure the shadow have the requisite numbers to accomplish this on such a grand scale. Trollocs perhaps, but the organisational structure above to do this is just not there.

2. Point two, Taim. And his surly, arrogant, aggresive male channelling buddies. If what you say is true and Demandred has another Black tower in the blight, which would be cool. I would anticipate these two factions, Taims lot and Demandreds lot actually fighting each other and rending the black tower in blood and fire rather than them all falling into line behind Demandred. Can you see Taim being subordinate to anyone? He knows he's pretty much a match for Demmy One power wise, and in the chaos of these last days he cannot ask for a better opportunity to carve out his own empire.

So, to sum up my rambling. I think that 1, the attack on Camelyn is one of the shadow's big plays. Camelyn is key. It's not just an example of what is happening everywhere else.
2. The dark male channelers will shoot themselves in the foot and turn on each other, thus scuppering any big plan for Demmy's world domination. After all, isn't this a central theme of the books? Evil cannot triumph basically because it is too self centered.