I did rewatch and it was exactly Robin who suggested that the Injustice League were The Light.2) In the Episode "Revelations" Aqualad said that the Injustice League was the "secret society" that they were looking for. It is then revealed that the Injustice League were just proxies used to divert the attention of the JL and YJ. Since Aqualad is my proposed mole here, it makes sense that Aqualad, being the rat, lead the League and his team into thinking that the Injustice League were the Secret Society to help the Light accomplish what they were trying to accomplish (diversion). Rewatch the episode and you'll see what I mean.
"...Seven heavy hitters, probably behind everyone and everything we've ever faced!"
All Aqualad offers is some opinion backing Robin's theory and a bit earlier he along with everyone else realized it's clear that super-villains are working together. But he didn't initiate the speculation and he didn't lead them to that conclusion since he was the last one to talk, and by then everyone was already on the same line of thinking.
Really? If anything that episode showed Superboy isn't all good. The guy was at peace when he thought everyone he knows and loves was dead.4) To me, the Episode "Disordered" was pretty revealing. In it you learn that Superboy is basically good (despite his odd need to be Superman) and to me that episode rules out that he is knowingly the mole as you see his motives to want to be good.
Red Arrow thinks Aqualad, Robin and KF are above suspicion since the four of them have been doing this for at least 2 years before The Team was even formed, and he knows them all pretty good. He isn't at all familiar with Superboy, Miss Martian and Artemis though.5) There's much more but also in "Secrets" Red Arrow says that Kaldur is "above suspicion" I don't really know what Red Arrow's basing that off besides the fact that Kaldur learned that there was a mole first...which would definitely play right into his hands. Also, as we so often learn, those "above suspicion" usually are the ones we should be suspicious about.