thanks for your statements and the photo, this antenna is imposing.
I think i understood it right: disabling the diversity means to switch off the onboard antenna in this case. I don't know if you have done some measurements but in my opinion the internal (onboard) antenna should be switched off 'automatically' as soon as an external antenna was connected and that's a pure hardware thing. Maybe i'm wrong, but that's what i've seen in other hf equipment with additional external connectors so far.