They are still Clemson. Take a breath and let the season play out.
But I'll play along. Looking at your scenario, in which the Big 12, ACC and Pac 12 are all shut out from the playoffs, that presumes that Oklahoma and Clemson lose a couple of games. Maybe not likely, but let's go with it.
So we are left with, presumably, Ohio State, three SEC teams and ND for the 4 spots. Let's say OSU has at most one loss and win the B10. They are in. Same for the SEC championship game winner. Let's say the SEC championship game loser has only one loss, so they get in too. So then it comes down to an 11-1 ND and an 11-1 or 10-2 SEC team that did not play in the SEC title game.
Like it or not, this is where the 'eye test' comes in. Let's say we win out, but struggle in a couple of games like we did last year against Ball St and Pitt. In that case, I don't think we'd get in. But if we not only finish 11-1 and also kick some major ass along the way, I'd think we would get the 4th spot.
Also, 3 SEC teams in the playoff might cause a US civil war, and I don't think the selection committee wants that on their resume.