Not sure about this and please give it a try. You can get EGLImage by calling NvEGLImageFromFd(). And if the OpenGL APIs ca work with EGLImage, it should work fine.
CUDA postprocessing is verified in 08_video_dec_drm. The function calls in cuda_postprocess() in12_camera_v4l2_cuda can work fine if you port it to 08_video_dec_drm.
Not sure about this. If it it similar to Cairo, it should work.