I don't know the specifics about the Garmin crossover cabling setup.
But assuming the "factory" installation
requires one proprietary cable from the MFD to the scanner, and it is a crossover cable, then:
1. You should have only one crossover cable in your OCPN installation
2. Probably sensible to make it the lower right cable in your drawing, probably the longest cable that came with the scanner.
3. The other cables may all be straights.
4. You can test this by connecting everything (without the Pin-5 switch installed), and prove that the MFD can see and control the scanner through the hub without OCPN involvement at all.