She made her arrangements with Myrelle (to pass her Lan's bond at death), before leaving Tar Valon to go to the Two Rivers. She was reaching the end of her quest and knew the risks involved. She didn't want to condemn Lan to death, so she arranged to save him. I suspect that in Moiraine's reasoning, the fact the War against the Shadow had to go on played its part. She wanted Lan to have his chance to keep fighting his war at the side of a Green sister if she fell herself before or during TG.

She chose Myrelle because Myrelle is the best at saving warders whose Aes Sedai died. Myrelle was a close friend and Moiraine could count on her discretion.

As for the arrangements for Myrelle to pass the bond to Nynaeve after she's saved Lan, it's fairly obvious. It came later, a change to the previous arangements. It took place in the Great Hunt, after the attack by a draghkar in which Moiraine nearly died. Moiraine had just had her oblique discussion with Lan about the passing of his bond to Myrelle, fishing for his reactions/thoughts about Nynaeve at the same time. She was about to make her decision (or had made it), and then the Shadow tried to take her out - it couldn't really wait anymore.

At the end of that chapter, Moiraine prepared to write letters the Namelles would send to Tar Valon for her. One of those was undoubtedly for Myrelle, telling her about Lan's bond and Nynaeve, a woman that was coming with Siuan to the Tower to be a novice (sic).