The FSDT KLAX installer doesn't care for the registry, and doesn't care for the scenery.cfg, it simply check if you have a Cloud9\LOSANGELES folder in your FS9 so, fact the installer is seeing it, seems to indicate you still have Cloud9 KLAX installed, maybe you just disabled it in the Scenery Library.
If the proper Uninstaller for Cloud9 KLAX doesn't work, it should be ok to just remove the whole folder from FS9, and FSDT KLAX installer should work.