It turns out that there're a lot of problems with interlaced videos here... 
1. SVP replaces the entire video filter chain with the vapoursynth filter, while interlaced videos need the yadif filter to be inserted before - luckily this's quite simple to fix
2. SMPlayer sets incorrect mpv option when using "Yadif (normal)" - it sets "--vf-add=yadif" instead of "--vf-add=yadif=frame", so both "Yadif" modes in SMPlayer now output doubled frame rate
3. Vapoursynth filter inside mpv can't properly handle video deinterlaced with doubling frame rate because the video IS actually 50 fps while the internal variable "container_fps" is still set to 25 fps.
So... to make interlaced video work with SVP we need:
1. Fix SVP Manager
2. Force SMPlayer to add "--vf-add=yadif=frame" to advanced mpv options