3) Remove ORBX Iceland Demo & Re-Run AEC AutoConfig
That's the mistake. In ALL posts about OrbX, nobody suggested to run Auto Config. In fact, that's precisely the problem. You must disable the airport MANUALLY.
4) At all times the GSX Customization Tool identified the AFCAD as ..\0501\scenery\APX41090.bgl
That's the problem. The AFCAD is the right one, so GSX correctly read this, but OrbX has installed another AFCAD that changed the elevation only so, it's not an AFCAD which GSX can use (there's nothing there, except the new elevation), hence the altitude error.
By disabling that airport manually in the AEC tool, that elevation correction AFCAD will be removed, so the airport will go back to its official published altitude, which is the one in the original AFCAD that GSX reads.
5) I checked FSX/P3D versions for this airport in ADE176 and the airport elevation is the same at 170.997 whether or not ORBX is installed on the sim (I have both clean and fully configured sims to compare).
That's because ADE, just like GSX, reads the AFCAD for the airport, not the other altitude correction file.
I also tried the editor. I could select the object but there appears no way to change the object's elevation although page 37 of the manual may suggest that this is possible.
The only objects that can be raised/lowered are the parking system (marshaller or docking system). The keys used by the editor while editing a Jetway are at Page 42 and it's not possible to change a jetway height. And, in any case, it wouldn't be the correct way to fix this problem.