By accident I now found the requirement for couatl.ex to run successfully: I had to set "run as administrator" in the P3D executable shortcut as well. So now it works and you could consider to update your guide in the "how to install" thread by adding "verify P3D is launched as administrator".
That's not how it's supposed to be. For some reason, P3D required to be run as admin on your system, but this has never been a requirement, and there's nothing in our software that requires admin access and it's not how we test it. Neither P3D nor Couatl.exe normally require to be run as admin.
The only possible reason it's required on your system, might be related to some changes in the default security settings in your own account. If it works for you, that's fine...