After my frustration with Vivado, I've done a discrete logic implementation of the "MVP".

A hub with 4 connections, with 4 switches, so it's essentially 2 inputs and 2 outputs. Each input can be switched to any one output, no output, or both outputs. That's enough flexibility to get some control software started, and plug in some real modules for testing.

Full schematic on github.

Next: layout, ordering parts