I think Moghedien might have communicated the series of events incorrectly. She did, after all, rely on Moridin to tell her what had happened to Lanfear.

We know that Moridin came to look at Moiraine and said that it was not her. He then went to Lanfear, who was being drained by the Finns in a similar way. I think that adequately explains the loss of her strength, though it remains curious as to why this loss follows through after death.

Moridin was probably unwilling to bargain for Lanfear, knowing how treacherous the Finns are. He probably killed her as soon as he saw her and then fought off the Finns before leaving himself. That was probably his plan from the beginning.

I don't think his plan was much deeper than that really. Moridin doesn't strike me as a man who would go through too much effort to save a Chosen if the Dark One can snatch them after death and place them in a healthy body.

I fully realize the symbolism of destroying the Lanfear persona, and it follows through with Graendal. Moghedien thinks that the DO made Graendal ugly for the same reason, but its equally likely Shadar Haran tortured her to death and they put her in the next available body. I'm not sure if the symbolism of destroying their images is more probable than the theme of immortality and how the DO has permanent access to their souls that extends beyond death.