I'd say, all the "remaining" areas - like edges, cracks, tiling imperfections - could be... just, you know, painted white high reflectance paint that'll be soon covered in dust anyway? This whole technology looks like a few % optimization over widespread and long adopted "white buildings are cooler in hot environment" thing for hype and whitewashing. Sure it's worth doing in punk DIY setups.

Not that I'd adopt that, I still live in cold climate, even with all the warming, I'm going to burn black all non-greenable and non-solarable surfaces black to get some extra heat energy locally.

If you are living in an area where it is cold in the winter, having a cooling coating on your roof is probably not the best use.

Every roof is not proper to get solar PV, it can be shaded for example. And you can find many application besides roofs : cooling vehicules like buses or trains or bus and train stops. Current applications are used also for technical appliance : pipes, outside electrical closet...

PDRC also comes as fabric, it would be possible to use it to cool down a small street between to building, or make a cooling hat, or a folding awning for houses, a car cover...