I trust your C / KC test, but you still haven’t proven to my satisfaction that this can’t be P. detersa. :) I’m arguing that all of your marginal lobes have no soredia, and that this points more to P. detersa than P. perisidiosa.
As for deciding whether it has cortex below or not, the key is the texture. The cortex of Physciaceae species is a layer of densely-packed hyphae parallel to the surface. Some are long and narrow (prosoplectenchymatous), others are short and blocky (paraplectenchymatous – I didn’t come up with these terms!), but in either case the result is a shiny, smooth, plastic-like texture. (At least where not overlain by a layer of pruina or tattered dead cells, making the upper surface of Physconia appear particularly dull, for example). The medulla, by contrast, is a very loose network of thread-like hyphae with mostly open space, having a cottony or cobwebby appearance under the dissecting scope. Heterodermia is the best genus to study the feature, because it spans the entire spectrum.
H. leucomela and H. erinacea with no cortex at all
H. neglecta with a hardened but still non-corticate lower surface
H. granulosa with a technically corticate but (I think) ambiguous lower surface
H. albicans and H. speciosa with a “normal” thick shiny cortex
Unfortunately I don’t have any photos of the underside of P. perisidiosa, but it is somewhere between H. neglecta near the tips tp H. granulosa toward the center. At least the few I’ve seen. It’s much less common than the other species (P. enteroxantha, P. leucoleiptes, P. detersa, P. isidiifera).