I've spent some time trying to solve the external display problem myself.
If the MXM slot is empty, then my UMA config works perfectly and external displays work.
But if there's a DGPU (turned off) in the MXM slot, then something changes in acpi and the mux chip still defaults to the dgpu.
Both windows and Linux don't have this problem, so I thought it could be a generic or at least very common acpi switch, but digging deeper it looks more like it's not a switch but rather how the os requests the screen that's the problem.
I believe I have found at least one of the mux control variables, but I can't seem to find a way to force the switch, or make it stay. Or even if it's the right mux chip. (There's at least 2, one for the lcd and one for external displays)
@EdwardGeo and I have been working on this problem as well as the WX card working on Catalina, but so far it's been a lot more involved than expected. The laptop has a lot of security features that block progress one way or another.
The only "workaround" so far is to plug external screen, press power button and close lid. The main screen will switch to the external display.
Problem is you don't get the LCD screen back, there's no hdmi audio, and only one external screen.