I've thought of another off-the-wall suggestion - but it's slightly disruptive, so you may not wish to try it.
The odd thing is that the NAS is happily connected to the camera and recording video from it.
But nothing else, especially your PC, can see any evidence that the camera exists on the network.
You have nmap, and SADP, and the browser on the PC. I'm assuming the camera has a static IP address.
My suggestion is -
Unplug the camera from the LAN, unplug the PC from the LAN, connect the 2 together. Ideally with a separate switch, but if not, directly together.
Change the PC IP address to be the same as the IP address the NAS is using.
Try SADP, try nmap, try the browser.
Change the PC IP address back and put the wiring back as it was.
The idea being to see if the PC having the NAS IP address changes the ability to access.