In regards to the second question. After having looked in more details at what is provided, or rather what is missing, the headers (and resulting implementation) is missing many vital parts of the Foundation (e.g. NSRunloop.hpp) and AppKit (e.g. NSEvent.hpp) frameworks. It is however possible to extend the headers yourself, with some effort.

Please ignore the code example above. While it produces the "objc[29459]: class `NSKVONotifying_AVCapturePhotoOutput' not linked into application" in the environment mentioned in the first post, the code itself is not correctly implemented for capturing a photo. I found a Swift implementation on Github that produces the same message, but works. I made a more robust example on my own and also made it work (I apologise for not being able to share it). In short, the message might (I still do not know) not be harmful. The example above is incorrect, I'm guessing because of ill managed lifetime and/or because it does not take into account the setup time needed for the camera to become ready for capture.