OK, so I've just been right back to basics again. Followed all of your installation instructions exactly, including checking the registry, uninstall, sort out the antivirus and then reinstall.
I used a clean download from the website. Installation seemed to go correctly, although a SODE dialogue box appeared, not sure what that was for.
Then started FSX SE and it crashed on start. So started it again and it came up with the warning about the bglmanx.dll. This now happens every time I start FSX SE and if I say run it, FSX SE crashes. So I say no, don't run it and FSX SE is fine, but no COATL or GSX in the addon menu.
So I checked the registry again and the FSX SE 10.0 folder has reappeared! It has a sub-folder FSX SE\10.0\Launch.
There must be something going wrong here but I've no idea what?