Yes, Ricoh fixed their firmware a few weeks after I did inform them of the bug, and since December 2017 the spatial audio soundtrack is correct for Youtube (Ambix 1st order, 4 channels, ACN/SN3D).
So, after having converted the equirectangular video from MP4 to MOV using the Movie Converter app from Ricoh, the file is ready for Youtube, without any other editing required.
And yes, reducing the video resolution to 1920x960 does not affect the audio quality (but results in a video which is not enjoyable on an HMD VR visor).
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