I believe thom can channel for a couple of reasons. First his name, Merrilin. This seems to be yet another reference RJ has lifted from King Aurthur's lexicon. Merrilin is Merlin. Furthermore, he was related to a male who could channel. It would be totally within Merrilin's character to guard this secret tighly. Think about it when you read, it makes sense. I also had simply assumed Aran'gar and Osan'gar were Lanfear and Asmodean. Both die at the end of TFoH and then in the beginning of LoC Arangar and Oshangar are intorduced. I believe that the Dark One, being very funny, switched their sexuality, with would explain why no one could feel a woman channeling in Salidar when Moghedien was released and why Egwene thinks that Logain freed her.