Making the Case
"...and Myrcella will become our guest at Winterfe-"
"NO! A THOUSAND TIMES NO! You won't have my daughter!"
Had Ned been challenged to say one good thing about the adulterous queen, the fact that she seemed to honestly love her children would probably have been the first (and only) thing to come to mind. Even so, he was still surprised by the vehemence of her response.
"I'll tear down your little northern hovel with my own hands before I let you have any of my children! I'll burn do-"
He could use this.
"If Stannis takes King's Landing, what will happen to her?"
"- what?"
"If Stannis takes King's Landing, if you are utterly and totally beaten..." he could see her bristle at the scenario outlined "...what will happen to Myrcella?"
The Queen remained silent.
"If Renly takes King's Landing, what will happen to Myrcella?"
Again, the queen silently stewed. Truth be told, Ned wasn't actually entirely certain what either Stannis or Renly would do with Myrcella if she should fall into their hands. But a woman like Cersei, who had no concept of honor or mercy herself?
She would no doubt assume the worst.
"You know I'm no slayer of children." he continued, "I was willing to let you escape into exile just to spare their lives. At great cost to myself."
He could almost hear the Queen thinking "you fool" within the privacy of her own head. But the point was well made.
"Myrcella will go to Winterfell. I will rest easy, knowing that you will not betray me while she remains my guest. And you will rest easy... well, easier in any case... knowing that she is safe, sheltered by the one enemy you have who will not harm her."
"And if Stannis wins? If Renly wins?" she asked, and he knew that he had won.
"If Stannis wins..." he said, and he paused, slightly amazed himself about what he was about to say and the fact that he'd actually mean it, "...,if Renly wins, it may be that they will write a letter to me, demanding I surrender her to them. And should that happen, I will respond with a letter of my own, assuring His Grace that of course I will turn over the Lannister spawn at once. And some time after that, there will be another letter, about how your daughter and her escort vanished without trace while crossing the Neck - slain by brigands and lost to the swamp, no doubt. And Stannis or Renly will have to accept it, because they will have no proof that's not what happened."
It would be treason, in a way. But he no longer cared. Not after all that had happened, not after what House Stark had lost and what they had almost lost... might still lose.
Honor was important. But it wasn't everything.
Stannis? Stannis had known, Eddard was sure of it now. He had known what had happened to Jon, he had known about the Queen's incest and adultery. And instead of warning him, so that they might set things right together, Stannis had thrown Ned to the lions while he sulked on Dragonstone.
Renly? Renly had fled the capital while Ned was struggling to keep the realm together and avoid a Lannister coup and civil war, he was trying to become King even though he had no right at all. He was no different from the Lannisters himself.
To their seven hells with them. He'd bend the knee, he'd call either of them "Your Grace" and pay the taxes if that was what it took to keep the North safe, but to their seven hells with them anyway.
"And Myrcella?" asked the Lannister snake in front of him, momentarily not a snake but a concerned mother. "Where would Myrcella be?"
He shrugged. "Wherever she wishes to be. Maybe the Free Cities. Maybe Oldtown. Maybe somewhere I've never heard of. She won't have a crown, or a House, or even a name, but she will have plenty of gold in her pocket, more waiting for her at the Iron Bank, and the freedom to go wherever she pleases. You have my word on that."