Why do you think Graendal remained in Arad Doman for this to happen? Why hasn't she wisely fled before him the minute he entered the country, as she had planned all along?

Because Moridin appealed to her greed and made her false promises that if she stayed and got involved with Rand, focussed on Rand, she would be rewarded and stand right below him (the best position from which she could topple him, as she saw immediately). Apparently, she hesitated and was careful and did basically nothing, but her greed for power still made her stay.. until Rand found her.

All the Forsaken who got too close to Rand with their schemes got killed.. Aginor and Belthamel, Be'lal, Ishamael, Lanfear, Ravhin, Sammael... they all died for it. Moridin is well placed to know this, he paid that price himself.

He set up Graendal so she would be there for Rand to kill her, and he gave Rand the key to destroy her. A very, very dangerous key - is destroying Forsaken worth the risk of destroying the Pattern using balefire? Weaving balefire with the CK was one of Rand's most insane actions in the whole series.

Destroying Semirhage and Graendal came at a heavy price. Both events nearly destroyed Rand - and by the end of the book he was such a ruin and so far gone he nearly destroyed the world, he came an hair from it, before he had his epiphany and made the right decision of never trusting anyone to use the CK again, the tool that could destroy the world before even the LB began.

That's how Moridin had decided Graendal and Semirhage could be served the DO, by being targets for Rand, victims that might shatter him. He set it up.

As always with Moridin's clever schemes, it came to his own grief in the end. Moridin is so inhuman, he doesn't understand humanity anymore. He thought to make Rand like himself, but he went too far and too fast and instead of shattering, Rand remembered love and once again denied Shai'tan and Moridin.

Moridin just made his own task even more difficult. Rand knows what he faces now, knows the perils of the True Power. What Moirdin achieved is making Rand the true Champion of the Light at last.