I can confirm i was getting the same terrain.dll errors with fresh install of windows 7, fresh install of FSX SE, FTX GLobal, FTXVector and FSGlobal Ultimate. I have since re-installed FSX SE and FSGlobal Ultimate and have been flying for almost a week without errors.
Not sure if this helps but i also have purchased the 747-400x just before the holidays and run into the same validation issue during install. While waiting for the response form PMDG support, I have come across a post that someone suggested changing the password on the PMDG website from complex to short and simple to resolve the validation problems; and yes it did worked for me as well...
Search for "effects_package_1.2.zip" in AVSIM library, AS ALWAYS backup your originals. After replacing the FSX default water spray i have never seen it trailing the aircraft in the air again. As an added bonus these textures are more realistic and much nicer looking than the default one. If your're interested in disabling it completely, checkout this post: http://forum.avsim.net/topic/274287-get-rid-of-snow-rain-spray/ Hope this helps