Unfortunately since we've put all our proverbial eggs into one basket, I'm now without 13 add-ons that I've bought and paid for because this scripting engine refuses to run.
The engine is not refusing to run. Something else in your system is causing this.
Fact that all our addons needs a single executable, it's obviously made for efficiency reasons, imaging the waste of resources, if each single airport needed its own .DLL (yes, some developers do that, which is really un-efficent), and in any case, even if we had different modules, chances are they will all stopped working anyway in your case.
If you think of some additional suggestions I would love to hear them
I've already made some additional suggestions, but haven't heard back from you about them: be sure that neither FSX.EXE or Couatl.exe have anything checked in their "Compatibility" tab of their Properties. That page should be entirely blank with nothing enabled, which is the default setting.
You might *TRY* a System restore to a previous date when you didn't had the problem, no guaranteed it will work because system restore is quite tricky and potentially dangerous, but it's the last resort before reinstalling Windows.
Another thing worth trying, is to create a new user account, and try install under that one, hopefully what is causing the problem is not installed for all users, and might not appear under a new user account.