Honestly, I don't think Jon even wants to be a ruler. It was thrust upon him. I think the only reason he accepted the position, is because he knows how serious the danger is. He wants to lead people in the right direction. Otherwise, I think he'd be content to let Sansa be in charge.
I think he'll listen to her, and weigh her opinions fairly, but in the end he will do what he believes is best for everyone.
Cersei could surprise us. She's trying to amass an army now with the help of Euron Greyjoy. She's hateful and ruthless and impulsive enough to send her army off to the north, to try to make everyone bend the knee. I wouldn't be shocked at all. I feel that they would be making a mistake to dismiss the danger she represents, because they're counting on logistics being a obstacle.