Time for an update! The system is up and running and behaving beautifully!
I ended up with three PID loops:
1: Room pressure, controlled by return air fan speed
2: Total air flow, controlled by supply air fan speed
3: Recirculated air flow, controlled by MD5
It wasn't necessary to control MD6 or MD10 at all; they are left at 100% all the time. The heat exchanger provides a non-insignificant restriction to the air flow, so as soon as I open MD5, air will bypass the heat exchanger even with MD6 fully open, because it's the path of much less resistance. So as long as my supply air fans are controlling the total air flow, and MD5 is controlling the amount of recirculation, then the amount of outside air flow must by simple math also be correct. And again, even with MD10 fully open, the supply air fans will draw air from the recirculation line if at all possible because it's easier than drawing it through the heat exchanger.
I set up the system so that each of the three loops would hold current output in manual mode until the previous loop was stable. Then I had each loop bias the one before it proportionally. So, the return air fans would start and establish room pressure. Then when the supply air fans start, a proportion of their PID's CO gets added to the bias of the return air fans PID, so that the return air fans ramp up in anticipation of the additional supply air load. Likewise, when MD5 starts to open, a proportion of its position gets added as a negative bias to the supply air fans PID, because the more you open that damper, the less hard the supply air fans have to work to achieve the same air flow.
Thanks for all the tips, between the 10-20 of us we got the job done. Good work team! Drinks are on me!*
*come visit me down under to claim