To answer my own question, yes, the MM can be used to access speaker level output for the left and right front speakers. For the 2020 86s AC LGD dongles, discussed below, are necessary.
As I stated earlier, the rear speakers are a little more difficult to access as they require one to use a digital multi meter to identify wires from the rear speakers to the head unit. These wires appear as different colors from rear speakers to HU (perhaps they merge with other wires).
AudioControl ("AC") tech support was very helpful in resolving the sound problem. They advised that I reposition the front speaker input from that outlined in the AC instruction booklet. This and by remotely operating the AC windows software (which was connected to the d-6.1200 via usb cable) the device was configured in such a way as to allow sound from all speakers.
Importantly, AC advised that many late model cars have stereo systems which somehow prevent a full level sound signal to be sent to non-factory speakers. AC is sending a set of LGD dongles designed to correct this problem.
If there is an interest, I'd be glad to post a DIY guide. Once the LGD dongles have arrived and the sound is impressive as I expect it to be I'd be willing to do so.
Now, if I can just get the reverse camera to display on the primary screen instead of the rear view mirror. I haven't yet found this listed on the DIY wish list though I suspect it is there.
I'd like to express my gratitude to Soundman98. Thank you for your patient guidance.
Also,
FR-S2GT86 and Laura at AC - your contributions to this effort have been invaluable.