Indeed, with your procedure it works well most of the time. But after a reboot I have to launch the camera from the lockscreen twice before it works as expected.

I tried before but with less manipulations and I still had to unlock the phone before. I assumed then that there is no need to unlock the phone with the stock app because the binary for the lockscreen is different than the on accessible from the app menu. But maybe it was because I messed a little to much with my phone.

I just tried to undo one by one the steps of your procedure to find what I missed, and it looks that with only "Exec=harbour-advanced-camera" in the "/usr/share/applications/jolla-camera-lockscreen.desktop" file, and after having restart lipstick or rebooted, advanced camera is still accessible from the lockscreen.