Thanks, we will. The pop-up message after sorting does say that you can view the Top-30 via View menu, but most folks (us included) click OK without reading it :)
Sorry about this: Very unusual. If the app can't access the library, presumably the 3+3 thumbnails on Home screen and the Top-30 report in View menu are also blank? Please reboot the Mac and try; if even that doesn't help please email us via the app mentioning your macOS version, so we can troubleshoot step by step.
Image Capture? It only captures images from devices like iPhone or DSLR cameras. As far as I know it has no sorting capability, and it also can't interrogate the Mac or iCloud Photos library - so how would it sort it?
Sure. As the alert says: Quit Photos. PhotoSort gets file size and other data from the same database that powers Apple's Photos app, so (in order to preserve its integrity) Photos 'locks' the database when it's using it. Closing Photos will solve the issue. Please confirm once it's resolved (or if you still can't).
Nope, everything is done locally on the device itself. Every item in the library is scored on various parameters e.g. composition, lighting, focus, color scheme etc. and the final ranking is based on a weighted average of these.