I am not sure there is a real 13th “tribe,” but there is certainly a 13th group of humans, on Earth. And one would have to presume elsewhere too, though I doubt we will see them.

As to who put in the block on the 7 Cylons, we have no idea, but I presume it’s the F5 themselves, or the Cylon God.

But they could have 2 Cylons to go instead of the “1 will be revealed” by using the Cylon Christ motif or other incarnations of the Cylon God. And in fact, there are really more Cylons than that. There’s Hera, and the hybrids, and the first hybrid whose “life will begain anew in ways uncertain.” And Nikolas Tyrol, too. Lots o’ Cylon models now.