The Wanderer (who helped Rand in Shadar Logoth), the watcher, and Moridin are all one in the same. Or so I believe. The reason why Rand could not detect the flows, and also the reason why the flows clashed is because Moridin taps into the True Source, not Saidin. Moridin helped Rand because, the Dark One said "Let the Lord of Chaos Rule," which I believe is some attempted plan to allow Rand to create situations in which some of the Forsaken can alter to aid the Dark One. Mazrim Taim, who I believe is Damondred, is certainly aiding Rand, while creating strife with the Asha'man. So why not Moridin aid Rand? There is a posibility that the Dark One wanted Sammael dead, and wanted to ensure that Rand killed him. Then again, it is all speculation...