FSX doesn't ask me to trust anything about Couatl and/or AddOn Manager.
This is not an automatic indication that something is wrong. You are not supposed to be re-asked for trusting a module if your previously installed version is the same.
To have FSX "forget" what you trusted, you MUST reply YES to the questions about removing the Addon Manager and Couatl the uninstaller does at the end. Only if you did that while uninstalling, and you don't see a Trust request made after a reinstall, you should start to worry about problems.
1) Enabled the Simconnect Verbose logger - NO GO. Doesn't show any error message
That doesn't mean much. In verbose/normal mode, you should also see messages saying the Addon Manager and Couatl are being loaded. If you don't see neither errors or loading confirmations, it means the sim is not even TRYING to load the modules, which leads to believe you DO have a registry problem, so the wrong XML file is being used.
3) Deleted the 10.0 Registry entry under FSX:SE to ensure that there was only the one entry (per Umberto's recommendation). NO GO.
That might not be enough.
Fact you HAD a wrong registry key you removed, already indicates your configuration is wrong, which is usually caused by defective installers (or wrong suggestions by developers who didn't care to support ALL the different FSX:SE modes of operations, so they told you to duplicate the FSX registry keys, which is wrong), but in addition to that, you must also check in which mode of operation FSX:SE is working, which has an effect on which XML file is used.
If your Co-existence key is set to 0, FSX:SE runs in "FSX mode", which is the correct mode of operations if you only have FSX:SE installed and no FSX. The 10.0 Registry entry under FSX:SE must NOT be present in this case.
If your Co-existence key is set to 1, FSX:SE runs in "Dual mode", which is the correct mode of operations if you only have both FSX and FSX:SE installed. The 10.0 Registry entry under FSX:SE must point to the FSX:SE folder, and the The 10.0 Registry entry under FSX must point to the FSX
AFTER you fix all your registry keys, you MUST reinstall GSX.