Well ok, so you need to modify the fusion sender (zed-unity-livelink/zed-unity-livelink-fusion/src/main.cpp at main · stereolabs/zed-unity-livelink · GitHub) by adding a function that checks if the cameras are detected (using getDeviceList) at the start, before starting the fusion and all the stuff.
I think you can ask chatgpt actually, and c++ is not that far from csharp anyway.