If you see both Couatl and Add-on manager menus under the addons menu, you can then safely exclude ALL the other possible issues you listed, because all of them would result in one or both menus not displayed.
If you are seeing both menus, but no scenery, the cause is not related to Addon Manager or Couatl, since they are running fine.
Check the following:
- Your FSX is not running into any of the "Compatibility" modes. This is not the default setting, and it shouldn't be changed.
- Your Scenery Complexity setting is set at least at "Normal" or better.
If this still doesn't apply, try this:
Open the Notepad and type the following on a new blank file:
logFile=couatl.log
(please note logFile is written beginning with a lower case l and a capital F )
Save the file as couatl.ini in the FSX\fsdreamteam\couatl folder, so it stays at the same level of the couatl.exe
Run FSX, go to KDFW, then quit FSX after the airport has been loaded. Check if you have a couatl.log file in the FSX\fsdreamteam\couatl folder and post its content here.