Thanks for the compliments! 
It’s an interesting case…
So, to expand on my first reply that only looked at the initial Extreme Music release but ignored the best-of release on EM and the general-availability release of “Master of Shadows”…
Technical answer: adjusting the relative volume of stems creates a new mix IMHO, hence each change would make for a new recording. But I wouldn’t assume that the variations on Extreme Music have been created by only moving faders on the multitracks, it’s possible that the officially released variations had more work done to them, as you’ve suggested as a hunch with one variant that seems to have pronounced crash cymbals which you couldn’t find in the stems.
For this specific case: Since it’s known that “Master of Shadows” (and probably other works by Two Steps From Hell) is available in multiple variations and the naming on the various releases doesn’t make it easy to identify identical mixes, would it be prudent to just consider each appearance of the piece a separate recording?
This would be along the lines of @jesus2099 suggesting to not merge recordings if we’re not sure in MB Community: To merge (recordings)… or not to merge?; his observation that all the recordings can be found as long as they’re linked to the same work as well as @IvanDobsky’s call for annotations would be gold in this case. I still think the workload to guess mixes shouldn’t fall on MB editors if publishers and labels put out variants but won’t be exacting in the information they provide.