I suspect I'll be carrying out a conversation with myself here for a while.

But what the heck. Maybe some other wacko will benefit down the line...
I removed the right-most 16/32 mixer (the "master" section) and ran all the 8 buses and 6 auxes into the topmost 48/96 mixer. Now the 32 input channels, 8 buses, and 6 auxes are all summed in a single master section.
Each of the 32 channels has 1 level control and 1 pan; the 8 buses each have 1 level and 1 pan; and the 6 auxes each have 1 level and 1 balance. Plus the master mix level. That makes 101 MIDI ccs for the master module (same as before, but with 1 less mixer plug taking up valuable resources.)
Thanks for the suggestion Ricardo -- it worked!
The 6 auxes now include 3 outboard effects devices and 3 new DSP ones.
I
was able to add 2 delays and 1 chorus as the 3 DSP effects.
I was
not able to add 1 delay and 2 choruses as the 3 DSP effects. This approach led to the same "out of DSP memory" error that I mentioned in the earlier post.
My suspicion is that those 32MB of onboard memory are used for delay effects. Whereas each "DSP memory" is used as a buffer for "shorter" processing.
One strange thing: the 3 external effects returns produce signals in the Aux return channels of the 48/96 mixer, as they should.
But the 3 internal DSP effects do NOT produce any signal!
Maybe this relates to
the thread which Rogurt started? Ralf, if you happen to read this, I suspect that Rogurt's problem is
only for internal DSP effects inserts -- not for hardware audio inputs into the return channels. (I haven't gone through all the steps to test this hypothesis though.)
I've also had trouble getting the Aux returns to keep their MIDI ccs. When I assign via the "Channels" window they're lost immediately. When assigned via the "Channel Strip" window they seem to be kept. I *think* it's just a cosmetic thing but haven't fully tested yet.
Cheers,
Johann