Since the installer downloads everything from the server, why is it installing an old version?
Because the antivirus is blocking the download of the new version, so you are stuck with the previous version you had before. As I've said, the version you have hasn't been online in months, we had several version that followed it.
You said you downloaded the full installer: that version is NOT even included in that installer so, even if you had no internet connection, so the Update didn't run, you should have a newer version, the one included in the installer.
How come the program is blocked by antivirus when the antivirus has been disabled?
That's because you are assuming you could disable the antivirus. The antivirus is not really disabled, because its low-level service is still running in the background so, the only way "disable" it would be uninstalling it.
That's why we NEVER, ever, suggested to "disable" the antivirus but, instead, add the whole Addon Manager folder to the Antivirus exclusions before running the FSDT Live Update.
Another thing you can try, is to manually REMOVE the Addon Manager\Couatl64\couatl64_P3D.exe file and run the FSDT Live Update again, and see if you get the correct version, maybe there's a permission problem that is preventing the file to be written over.