Moridin as Ishmael at least was pretty successful in driving the course of history over 3000 years towards the Dark One's goals, helping the overall countries slowly deteriorate, and have Hawkwing's descendants return just at the wrong time for the forces of light.

Once Rand and company started becoming player on the chess board it became more difficult, as he was fighting their Ta'veren natures and the wheel / pattern more at that point.

Regarding being duped, in a previous post I mentioned that he didn't realise the Dark One's end goal, but thinking on it more, Moridin's goal was for oblivion, to not be reborn again and again to fight the champion of light (as I believe he is fated to do), and so I think deep down he probably knew that the Dark One wouldn't let him go, even if successful in breaking free, but it was his only hope for oblivion (outside perhaps of Balefire, but I suspect that the likes of Rand, Moridin and the Heroes of the Horn likely can't be truly Balefired).

Regarding Graendal, she was pretty effective compared to the other forsaken when it came to the various subplots of the Dark One, and during the Last Battle, however as Moridin and Rand at least realised, and potentially Lanfear as well, but the other forsaken didn't, winning the Last Battle wasn't going to free the Dark One, and so wasn't what the Dark One was truly after, it would just put the world in darkness, making life harder for the light in the next age, but not ending the turn of the wheel. Thus they wouldn't become truly immortal, and the Dark One would happily sacrifice them at a later date in order to try and win the next confrontation with the champion of light. I would imagine that over the many turnings of the wheel, the Shadow's forces have won at times, but the Dark One has yet to break free, and so Light has also turned things around and defeated the forces of darkness in later ages.

Thus Graendal was effective, but only in terms of winning the battle, not the war.