I think the GoR stays in effect until its current expiration

Date. As I understand it, any modification requires unanimous agreement. I also happen to think there is zero chance that it gets extended, as the unanimity requirement cuts both ways.

As for the possibility of leaving early, consider that Oklahoma and Texas had to pay about $100 million (not sure if that figure is combined or apiece) to get out of the Big XII's GoR just one year early. Since ND football is a separate entity, ND likely wouldn't have to pay that much, at least not proportionally, but it still could get pretty expensive.