Interesting thoughts Barry, and I agree that it is a de facto key change to A flat major at RN 213 (though, what happens next is beyond my grasp of amateur knowledge of harmony). Mahler even divides the choruses to make it chord with a nice full sound of it. And since the orchestra is silent at this crucial point, if not else, this alone should make it a priority for an audible organ, just as you said.
As for the terraced dynamics up to the end, that is indeed a very important aspect that gets easily overlooked (just when I think I get overwhelmed by the sheer joy of an Eb major with the Accende- and Veni-motifs it gets even louder!). I remember when I started looking at scores and reading more about how composers notate music, and how much work there is in thinking about and creating the music from the paper. Considering that it's easier to hear the highest and lowest pitched parts in the orchestra (and the piercing sound a trumpet has at ff), I do indeed wonder about the Mahler's "mehrfach besetzt" indication.
I'm not audio engineer, so could it be, that it's just impossible to contain such a wide dynamic Mahler's 8th requires?
P.S. The Dover score is a reprint of the 1976 Russian publication by 'Izdatel'stvo "Myzika"' (Moscow). Though I wonder on which edition they based their print, considering that there hasn't been any "critical edition" of the 8th yet, it should be Universal Edition.
