Interesting post, as a Godzilla fan, I’m interested in this, as well. I’m actually doing a project reconstructing in the style of Red Menace the International EOST (English On Screen Text) version. While I am not finished because I don’t have a final textless source for a few select shots (which I wouldn’t mind help with 😉), I do know that there are actually three Japanese audio mixes. One, the Japanese theatrical Stereo mix (the “A” mix), the Japanese mono mix, labeled the “B” mix, which was used in some theaters but mostly for home video, and the recent 5.1 upmix which we have on the Japanese BD.
Then there’s the 1985 American version, you’ve covered all of the audio versions.
For the International version, there’s the mere dub we have on the BD, but that dub is actually just the Japanese 5.1 upmix with the Japanese dialogue stems switched out with the English ones. If you listen closely, there’s subtle differences such as grunts that are in the original mono English Int’l dub that aren’t in the 5.1 upmix on the BD. That dub originally was mono and was used as an export version, which can be found on the Carlton VHS, which has the dub, as well as the English titles (which recreating them in HD is the goal of my project).
Hope that helped!