Could you tell me exact files fsdt installers change or write into fsx and windows? I would like to attempt cleaning it completely.
Not much, really. Apart for the scenery files and other couatl support files (that being python programs running outside FSX in the couatl interpreter, can't create any issues to FSX), the only files installed in FSX are:
The Addon Manager .dll and related .dat files ( bglmanx*.* in the FSX root folder )
Outside FSX, the files installed are:
the Esellerateengine.dll in the C:\ProgramData\Esellerate folder
And then our installer launches the official VC++ 2005 redistributable installer from Microsoft, which will install the 2005 runtimes but, if the correct version is already found on the system, it will not do anything (that install step will be very short)
I started suspecting pmdg stuff as it also uses c++ libraries, but I simply still can't find the culprit.
PMDG uses the VC++ 2008 runtimes and yes, there was a known problem (also acknowledged by Microsoft) that installing a specific version of the VC++ 2008 runtimes over an existing VC++ 2005 runtimes install, would cause a conflict, resulting in crashes in MSVCR80.DLL. I believe Microsoft should have fixed this a while ago, but I'm not sure exactly which version was the affected one. We purposely use the 2005 version, because it's the one that FSX SP2 uses, so the reasoning was that using the same version as FSX would minimize conflicts.