Thank you very much for sharing your experiences!
Generally, it is not recommended to install multiple versions of Grasshopper-Archicad connection. However, we can bypass this limitation by activating a developer setting:
In Rhino there is a command you can enter: "GrasshopperDeveloperSettings". It allows you to specify the directory where "ArchiCADConnection.gha" file is located in. You can add multiple locations here. Upon starting Grasshopper, if it finds multiple ArchiCADConnection.gha file, it will ask which one you want to load.
For Archicad, it requires the correct APX file inside Add-ons folder.
It's not an easy thing to execute, this is why we don't recommend using it.
I hope this gives you an idea of how to have multiple Grasshopper-Archicad connections.
Technical Support Engineer