"How do we know that the individual inducing elements are indistinguishable from the camouflage elements? What about pairs of elements? Would local edge alignments constitute local features or global features?"

As explained in the paper, this can be verified by observing isolated regions of the figure. The triangular perimeter cannot be seen. Furthermore, it is impossible to devise a computational algorithm that would distinguish the significant edge of the triangle based on these local samples- even the full power of the human visual system cannot do that from local views of the stimulus.