Maybe I'm naive, but I don't think it's possible for ND to get left out.
I think JS has had this in mind for a while, and part of the reason that the B1G/NBC deal is important. ND and the B1G both have a convenient "excuse" for ND to join if ND's independence is no longer practical for playoff/bowl game purposes. If the B1G is at how ever many schools... they'll add ND and have an uneven division/pod, or they'll find a second team in the South East to add to expand the footprint while keeping the divisions/pods even.
ND *probably* has an invite to any conference it wants, whenever it wants. If they want in the B1G bad enough, ND will do whatever it takes to make it happen.
If for some ungodly reason they want in the SEC after all the dust settles, the SEC will do whatever it takes to make it happen.