If I do it, they'll both be converted into Avid DNxHD anyway, and I'll handle the framerate conversion and color correction within Avid.

I'd just use the Japanese one's audio, since Avid can't work with 5.1, and I wouldn't need to change the audio anyway. My video can just be muxed to the Japanese's 5.1 audio.

Then I'd export a master (possibly DNxHD or ProRes) and use that to make MKVs, AVCHDs, and whatever else would be desired.

The conversion into Avid would bring it to a higher-quality, higher-bitrate format, so very little, if any information will be lost. Same with the export master, and all rendered effects (color correction) within Avid.

I just need the files to work from and I can do this (I think) rather quickly.