Just a quick bit of input on this - I think I'm reading this right.
Having just installed the Holley EFI system in the Midget, consistently in the documentation, they warn against grounding the efi-ignition ECU to a common ground and suggest that the best practice is to take an individual ground lead and individual hot lead directly to the battery. This is to provide clean, non-noisy power to the ECU and minimize potential miss-reads.
Now you and I know that by doing this, we have made a "common ground" at the battery terminal, but what I think this layout does is assures that if there is a ground error in the ECU, it makes it easier to trace, and if there is an error in the rest of the ground system, any spikes or drops will be, in part, somewhat absorbed by the capacitance of the battery before it effects the brain box.
Additionally, according to Holley, one risks frying the ECU is you accidentally crank the engine with a noisy battery charger hooked up.
It's so important that Holley won't warrant the box of you take it to a common ground.
Seeing as you've got two complete circuits to deal with, I'd say isolate the one with the most sensitive electronics, and minimize the common ground by taking a direct wire from the block to the negative post of the 16.5 V.