[StainlessS]

bugFix,

was

    Stereo = (nChannels>=2) ? Last.GetChannels(1,2) : (nChannels==2) ? MergeChannels(GetChannels(1),GetChannels(1)) : SilentStereo

should have been

    Stereo = (nChannels>=2) ? Last.GetChannels(1,2) : (nChannels==1) ? MergeChannels(GetChannels(1),GetChannels(1)) : SilentStereo

I'll leave it to you to perfect it.
Fixed in prev post.

EDIT: Your original Big Buck Bunny Link (downsized to 854,480)


And current script(approx same frame, needs some coords fixed a bit)



EDIT:

The idea is to pick both Left and Right channel when stereo and pick just Front Left and Front Right when is Dolby.

From Avs Docs

Every file format has a different internal channel ordering. The following table gives this internal ordering for some formats (useful for plugin writers), but it is a decoder task supply to AviSynth the expected channel order (like WAV), if you use decoders like NicAudio/BassAudio or ffdshow/AC3_filter you don't need to worry about this:

reference:      channel 1:     channel 2:     channel 3:     channel 4:     channel 5:     channel 6:
5.1 WAV         front left     front right    front center      LFE         rear left      rear right

So I take it that channels 1 and 2 should be correct whether DOLBY or not (as we are dealing with WAV order).